The first known written use of the word Britain was an ancient Greek transliteration of the original P-Celtic term.
It is believed to have appeared within a periplus by the geographer and explorer Pytheas of Massalia, but no copies of this work survive.
The earliest existing records of the word are quotations of the periplus by later authors, such as those within Strabo's Geographica, Pliny's Natural History and Diodorus of Sicily's Bibliotheca historica.
According to Strabo, Pytheas referred to Britain as Bretannikē, which is treated a feminine noun.

 "Britain" is most like Welsh Ynys Prydein, "the island of Britain", in which is a P-Celtic cognate of Q-Celtic Cruithne in Irish Cruithen-tuath, "land of the Picts".
The base word is Scottish/Irish cruth, Welsh pryd, meaning "form".
The British were the "people of forms", with the sense of shapes or pictures, thought to refer to their practice of tattooing or war painting.
The Roman word Picti, "the Picts", means "painted".

This etymology suggests Pytheas most likely did not have much interaction with the Irish as their language was Q-Celtic.
Rather, Pytheas brought back the P-Celtic form from more geographically accessible regions where Welsh or Breton are spoken presently.
 Furthermore, some proto-Celtic was spoken over all of Greater Britain, and this particular spelling is prototypical of those more populous regions.

Diodorus based on Pytheas reported that Britain is cold and subject to frosts, being "too much subject to the Bear", and not "under the Arctic pole", as some translations say.

The numerous population of natives, he says, live in thatched cottages, store their grain in subterranean caches and bake bread from it.
They are "of simple manners" (ēthesin haplous) and are content with plain fare.
They are ruled by many kings and princes who live in peace with each other.
Their troops fight from chariots, as did the Greeks in the Trojan War.
The first known written use of the word Britain was an ancient Greek transliteration of the original P-Celtic term. It is believed to have appeared within a periplus by the geographer and explorer Pytheas of Massalia, but no copies of this work survive. The earliest existing records of the word are quotations of the periplus by later authors, such as those within Strabo's Geographica, Pliny's Natural History and Diodorus of Sicily's Bibliotheca historica. According to Strabo, Pytheas referred to Britain as Bretannikē, which is treated a feminine noun.
 "Britain" is most like Welsh Ynys Prydein, "the island of Britain", in which is a P-Celtic cognate of Q-Celtic Cruithne in Irish Cruithen-tuath, "land of the Picts". The base word is Scottish/Irish cruth, Welsh pryd, meaning "form". The British were the "people of forms", with the sense of shapes or pictures, thought to refer to their practice of tattooing or war painting. The Roman word Picti, "the Picts", means "painted".
This etymology suggests Pytheas most likely did not have much interaction with the Irish as their language was Q-Celtic. Rather, Pytheas brought back the P-Celtic form from more geographically accessible regions where Welsh or Breton are spoken presently.  Furthermore, some proto-Celtic was spoken over all of Greater Britain, and this particular spelling is prototypical of those more populous regions.
Diodorus based on Pytheas reported that Britain is cold and subject to frosts, being "too much subject to the Bear", and not "under the Arctic pole", as some translations say.
The numerous population of natives, he says, live in thatched cottages, store their grain in subterranean caches and bake bread from it. They are "of simple manners" (ēthesin haplous) and are content with plain fare. They are ruled by many kings and princes who live in peace with each other. Their troops fight from chariots, as did the Greeks in the Trojan War.
Pytheas