Q:

Origin of the term "deadeye" meaning "expert marksman"?

The term deadeye means 

(informal, chiefly North American) An expert marksman

Oxford Dictionaries Online
(There is an apparently unrelated sense of the term referring to a specific type of nautical block.)
Numerous other online dictionaries give nearly identical meanings, and a few also list it as an adjective with an equivalent meaning. But none of these sources give an etymology.
A search on etymonline.com gives no results.
A quick review of ngrams shows the term being used as a name or nickname as early as 1829 and a character known as Dick Deadeye appears in Gilbert and Sullivan’s HMS Pinafore, but none of these 19th century cites seem to relate to a marksmanship characteristic. 
There also seems to be a series of dime novels featuring Deadeye Dick, but I cannot find any text to indicate if the name is associated with a sharpshooting skill.
So, can anyone assist with the origin of the term deadeye to mean marksman?

A:

You've referred to etymonline, but perhaps you need to think of it as a compound adjective+noun, not a simple noun:
http://www.etymonline.com/index.php?term=dead

Used from 16c. in adjectival sense of "utter, absolute, quite" (as in dead drunk, first attested 1590s; dead heat, 1796)

As such, someone with a "dead eye" or who was a "dead-eyed shot" had an absolutely sense of aim, and was thus an excellent marksman. See definition 6 of eye vs the literal squishy thing in your face:

the power of seeing; appreciative or discriminating visual perception:

Hence, "an absolute ability to see" or "an utter power of visual perception".
The previous etymonline link does mention shooting, but in the slightly different sense of:

Dead on is 1889, from marksmanship

... and this is probably the same root (as also in "dead on target", "dead shot") and chronology (as earlier guns weren't renowned for accuracy no matter how good a shot you were). The HMS Pinafore use could perhaps refer to cannon shot instead of blunderbuss usage, therefore (I'd be interested if there are any earlier bow-and-arrow references, as archers could be extremely accurate - hence the bullseye as in "able to hit a bull in the eye"/"the small red disc at the centre of a target"/"a small target").
https://www.oxforddictionaries.com/definition/english/dead-shot
http://www.thefreedictionary.com/dead+on+target
http://www.dictionary.com/browse/bull-s-eye
EDIT
More supposition... thinking about the nautical use of "deadeye" - an item used in the standing and running rigging of traditional sailing ships - I suppose it's possible that there's a convergence of meaning here. A "deadeye shot" from a cannon (not famous for their aim when onboard a moving vessel) could cripple a rival sailing ship if it hit that area

Single deadeyes (or bull's eyes) are used to guide and control a line and, particularly in older vessels, to change its direction

https://en.wikipedia.org/wiki/Deadeye
That in turn may also give rise to the "bullseye" as well, as it's a common link between the phrases: hit that bit, and your opponent is disabled. That's far more valuable than sinking a ship, as they were expensive things to be captured instead of left on the seabed.
Interestingly, that same wikilink states that archery does not refer to the bullseye, instead referring to the centre as "the gold", so that perhaps explains a lack of usage in earlier sources.

A:

I think this is related to the phrase "dead on," which is also used in marksmanship and comes from the fact that what makes a shot successful is often that it's fatal.
OED has:

Quite certain, sure, unerring. (Cf. dead certainty in sense A. 31a.) dead shot, one whose aim is certain death; so dead on the bird. dead-on: certain, unerring, exactly right (see quot. 1889). See also dead-hand n. 2.
a1592   R. Greene Sc. Hist. Iames IV (1598) iii. sig. F,   I am dead at a pocket sir..I can..picke a purse assoone as any theefe in my countrie.
1681   J. Chetham Angler's Vade Mecum x. 74   It's a dead bait for a Trout.
1776   F. Marion in Harper's Mag. Sept. (1883) 547/2: It was so dead a shot they none of them said a word.
1830   M. R. Mitford Our Village IV. 42: A silent, stupid, and respectable country gentleman, a dead vote on one side of the House.
1853   Dickens Bleak House xxvi. 261: With a gun in his hand, with much of the air of a dead shot.
1874   G. W. Dasent Half a Life II. 227 Those who do so..are almost always dead plucks.
1889   A. Barrère & C. G. Leland Dict. Slang I. 300/2: Dead-on (riflemen), straight on. A rifle-shot talks of the aiming being dead-on when the day is so calm that he can aim straight at the bull's eye instead of having to allow to the right or left for wind. He is said to be dead-on himself when he is shooting very well.
1959   Punch 17 June 815/1: She sang all night with pure, dead-on tone.
1966   ‘K. Nicholson’ Hook, Line & Sinker ix. 102: Don't you think a gesture like this is simply dead-on, when it comes to showing how with-it the Church is today?

For dead-hand:

colloq. An expert (at doing something).

1848   Thackeray Bk. Snobs vii. 31: He is a dead hand at piquet.
1862   G. O. Trevelyan Interludes in Verse & Prose (1905) 181: A young member of the Secretariat, a dead hand at a minute.
1888   ‘R. Boldrewood’ Robbery under Arms I. xv. 194: First-rate work it was, too; he was always a dead hand at splitting.

It seems like "dead-eyed" is a variant of "sure-eyed" using this sense of 'dead.'
Edit- Just to make this explicit: skilled marksmen are often referred to as "sure-eyed," "keen-eyed," "true-eyed," etc. because of the importance of vision in getting a good shot.

Edit 2: Bib asked how 'dead' came to mean 'total' or 'exact.' The 'quite certain, sure, unerring' sense I quoted first says cf. dead certainty in sense A. 31a, which is

Absolute, complete, entire, thorough, downright. Also dead-earnest in adj. use.
[Arising out of various earlier senses.]
1660   R. Sharrock Hist. Veg. 20   Till the seed..be come to a full and dead ripenesse.
1766   O. Goldsmith Vicar of Wakefield I. xii. 119,   I had them a dead bargain.
1805   Scott Let. 12 Apr. (1932) I. 248   This is a dead secret.
1842   S. Kettell Quozziana 47,   I saw, to a dead certainty, that if I should..be caught with my mouth open, I should be expected to say something. ...

The "various earlier senses" seem to be along the lines of 'completely still':
A.18:

Characterized by absence of physical activity, motion, or sound; profoundly quiet or still. (earliest 1548)

A.22:

Of water, air, etc.: Without motion or current; still, standing; (earliest a1000, a1552)

It is plausible to me that the sense involved in 'dead shot' arose fairly independently, though, because of the close connection between shooting accuracy and death.

This 1906 United Service Magazine has the earliest use of "dead eye" for a marksman I can find:

We all know what carnage can be inflicted if "the man behind the gun" will only keep himself cool and collected, alter his sights as the occasion requires, and aim with merely reasonable accuracy in the required direction. We do not require the "bull's eye shot" for this, but only a man who has been carefully drilled in the handling of his rifle, and can hit a third class target, say, at 500 yards. The Bisley marksman who can score twenty-six bull's-eyes running by the aid of verniers and wind-gauges is out of his element in war; our old friend the "dead eye," the marksman who makes such a wonderful score in the yearly courses, is little, if any, better than the much despised second- or third-class shot.

This early use of "dead eye" as someone who is really proficient at target shooting is really notable. It suggests to me that the term may be more closely connected to "bull's eye" than I expected—like "dead on the bull's eye" or "dead shot to the bull's eye."
Such an origin would also explain why I'm not finding any early "dead-eyed" references, which you'd expect if it came from "sure-eyed."
Edit: The use of "bull's eye shot" to refer to a person is also noteworthy: "We do not require the "bull's eye shot" for this, but only a man who has been carefully drilled..."

A:

I have not been able to find a concrete link, but in addition to all of the above, it seems to have roots and influences in nautical/mechanical terms, hunting/sportsman terms, and with the shift from outdoor shooting/hunting to target shooting/marksmanship, the shift from "dead shot" to "dead eye" happened sometime after WWI, and the generalization to athletic prowess and hand-eye agility occurred shortly thereafter in the decades that followed.  
1) deadeye or dead man's eye

(Google image) "deadeye" from "dead man's eye" = nautical term
From: "The Sailor's Word-book: An Alphabetical Digest of Nautical Terms "
by Admiral William Henry Smyth (1867)
-  "A sort of roundish flat wooden block or oblate piece of elm...The term   dead seems to have been used because there is no revolving sheave to lessen the friction..."
2) dead center  (also: "exact" = dead center of a target, or dead point: a center that does not revolve in a machine tool.

from Websters Seventh New Collegiate Dictionary 1971

3) dead shot
There are many examples of "dead shot" in literature: one as the title of the play "Dead Shot, a farce in one act" by J.B. Buckstone, an English playwright of the late 19th century, and a second, later example:  "He had always been afraid of Wonota. She was a dead shot and he believed that she would not shrink from killing him." from "Ruth Fielding in the Great Northwest Or The Indian Girl Star of the Movies", by Alice B Emerson, 1921.  There's also a character, an expert marksman, named "Deadshot" who is a member of the Suicide Squad. 
However, I find the most compelling evidence in a definition from "The Dead Shot, Or, Sportsman's Complete Guide, being a treatise on the use of guns..." By Marksman, 1863:

"The Dead Shot" - 
   He only can be called a "dead shot" who can bring down, with unerring precision, on October or November partridge, whenever it offers a fair chance, i.e., rises within a certain range.  No matter what the line of flight taken by the bird; whether it transverse, curved, rectilineal, oblique, or otherwise, to right or to left, and to or from the sportsman, if there are no obstructions, as trees or fences, the "dead shot" will knock it down and bag it; and no game is considered killed that is not bagged.  
This defninition, however, does not imply that every partridge which rises within range must be killed, in order to sustain the character of a "dead shot". If so rigorous a construction were put upon the explanation given, a "dead shot" would be a nonentity. A too sudden surprise, a slip or stumble, a rheumatic cringe, or some other unexpected occurrence, may cause the best shot to miss.  But making allowances for all such events, with preparation and expectation at his elbow, few indeed are the "fair chances" that escape a "dead shot".
Where game is abundant, and there is no scarcity of powder and shot, a great many birds may be killed in a day, but he who, with few birds and few shots, fills the game bag, is, at least, "a sportsman", if not a "dead shot."

4) dominant eye .... deadeye 
In marksmanship there is a concept of "dominant eye" sighting:  Dominant Eye or Master Eye: "No matter if you are shooting a rifle, shotgun or handgun, you will first need to determine your dominant or master eye. The "dominant eye" is the eye you use for sighting purposes. The dominant eye is the stronger of your two eyes." From IHEA (International Hunter Education Association - website page titled "Shooting Skills".  The dominant eye remains fixed, while the non-dominant eye shifts in order to focus and reduce the intrusion of ghost targets. 
From wordola.com, there is an abundant use of "deadeye", mostly referring to athletic acuity and agility, in a broad range of sports from basketball to golf.  However, I found most interesting a reference to "deadeye" in marbles: a way to shoot a marble with a top spin, (rather than a backspin), so that the top of the marble remains in one still position while it rotates as it rolls.  One marble player who became quite adroit at recess and won many valuables on the playground called himself a "deadeye" at marbles.
From a few examples in literature:
- From Tobias Wolff's memoir,  "This Boy's Life":   ...taught my mother so well that she became a better shot than he was — a real deadeye.
-From Jerome Klinkowitz 2012 "Kurt Vonnegut's America", in critiquing "Deadeye Dick", he states, " deadeye dick is more properly a nickname for a sailor...But a century later, in a landlocked part of the American Midwest, it has been applied more literally to marksmanship."
5) dead naught (nuts) / dead on / dead on balls accurate (an industry term)
From: http://forum.weldingtipsandtricks.com/viewtopic.php?f=2&t=1261 2011

"I use the expression " dead nuts on" on an almost daily basis, but I never once thought about where it came from...This...version i found on a website called "blurtit"...A bit of a combination of origins. "dead" as in "dead naught" or "dead plumb" has roots in nautical measurement and carpentry to mean "precisely". In this case "nuts" refers to the double zero "00" common on measurement and surveying equipment (non-electronic). If a measurement is precisely at the double zero, such as when closing out a surveying loop, this is the most precision possible with the given piece of equipment. You may also hear surveyors refer to this as "balls on" measurement for the same reason.  I use it mostly for measurement, like, that hole has to be bored out to 3.1232" and it has to be dead nuts on for a precise fit."  And of course, there is the film "My Cousin Vinny" with the famous line "dead on balls accurate"...

I am neither mechanically nor nautically inclined. I am not a bad shot for a beginner with my Ruger, but like most women, I have cross-dominance and will likely never develop a deadeye... In all, I agree with Sven.  His reference to "dead aim" is akin to "dead ahead" which is vessel-related, but someone has to do the pointing and shooting (or steering) right? 
Also, I believe that the HMS Pinafore character, who wore an eyepatch, has little to do with the term "deadeye" evolving into its North American usage.  The nautical "dead eye", with holes that had no moving parts, and the many nautical variations of usage for the term "dead", may have some loan-ish influence on mechanical use of "dead" as in unmoving and "dead" as is alignment in sportsmanship, among other influences.  But I think once "dead shot" became contextualized to land based hunting sport, it lost its nautical connection.  And from there, when marksmanship transitioned into target/shooting range sport (as the culture shifted from "bagging game" to "marking paper") the measure of mastery was less in the bird count and more in the target scoring, and so marksmanship technique focused more on the behaviors and attributes of the shooter, than the shooting experience as a whole; to be a "dead shot" in partridge bagging became "dead eye" in target shooting.

