The monkey's fist is a spherical covering with six surface parts presenting a regular over-one-and-under-one weave.
This weave is commonly doubled or tripled to present an appearance that superficially resembles a Turk's-head.
Like the Turk's-head, the knot is tied with a single strand, but here the resemblance ceases.
The Turk's-head diagram consists of a single line; the common monkey's fist diagram has three separate lines, which are best represented by three interlocking circles, in the best Ballantine tradition.
To tie a knot on this diagram with a single strand, it is necessary to complete each circle in turn—that is, to double or triple it, as the case may be—and when this has been done to deflect the strand into another circle which is completed in turn before commencing the third and last circle.

— The Ashley Book of Knots
The monkey's fist knot is most often used as the weight in a heaving line.
The line would have the monkey's fist on one end, an eye splice or bowline on the other, with about 30 feet (~10 metres) of line between.
A lightweight feeder line would be tied to the bowline, then the weighted heaving line could be hurled between ship and dock.
The other end of the lightweight line would be attached to a heavier-weight line, allowing it to be drawn to the target easily.

The knot is often tied around a small weight, such as a stone, marble, tight fold of paper, grapeshot, or a piece of wood.
However, this may be considered unsafe and therefore poor seamanship.

The UK Maritime and Coastguard Agency’s (MCA) publication “Code of Safe Working Practices for Merchant Seamen”, Section 25.3.2, states that “heaving lines should be constructed with a “monkey’s fist” at one end.
To prevent personal injury, the ‘fist’ should be made only with rope and should not contain added weighting materials”.

They should not be attached by metal or plastic clip to the heaving line.

Some port authorities instruct linesmen to cut off monkey's fists that use these fastenings.
The monkey's fist is a spherical covering with six surface parts presenting a regular over-one-and-under-one weave. This weave is commonly doubled or tripled to present an appearance that superficially resembles a Turk's-head. Like the Turk's-head, the knot is tied with a single strand, but here the resemblance ceases. The Turk's-head diagram consists of a single line; the common monkey's fist diagram has three separate lines, which are best represented by three interlocking circles, in the best Ballantine tradition. To tie a knot on this diagram with a single strand, it is necessary to complete each circle in turn—that is, to double or triple it, as the case may be—and when this has been done to deflect the strand into another circle which is completed in turn before commencing the third and last circle.
— The Ashley Book of Knots
The monkey's fist knot is most often used as the weight in a heaving line. The line would have the monkey's fist on one end, an eye splice or bowline on the other, with about 30 feet (~10 metres) of line between. A lightweight feeder line would be tied to the bowline, then the weighted heaving line could be hurled between ship and dock. The other end of the lightweight line would be attached to a heavier-weight line, allowing it to be drawn to the target easily.
The knot is often tied around a small weight, such as a stone, marble, tight fold of paper, grapeshot, or a piece of wood. However, this may be considered unsafe and therefore poor seamanship.
The UK Maritime and Coastguard Agency’s (MCA) publication “Code of Safe Working Practices for Merchant Seamen”, Section 25.3.2, states that “heaving lines should be constructed with a “monkey’s fist” at one end. To prevent personal injury, the ‘fist’ should be made only with rope and should not contain added weighting materials”.
They should not be attached by metal or plastic clip to the heaving line. 
Some port authorities instruct linesmen to cut off monkey's fists that use these fastenings.
Monkey's fist