Britain (place name)

The term Britain is a linguistic descendant (reflex) of one of the oldest known names for Great Britain, an island off the north-western coast of continental Europe. The terms Briton and British, similarly derived, refer to its inhabitants and, to varying extents, the smaller islands in the vicinity. "British Isles" is the only ancient name for these islands to survive in general usage. Its first written appearance was by Pytheas of Massalia in the 4th century BCE. It originates with a group of P-Celtic speakers, resident on Great Britain, who were referred to, and perhaps referred to themselves, by the earliest known form of the term "British".

"Britain" comes from Latin Britannia~Brittania, via Old French Bretaigne and Middle English Breteyne, possibly influenced by Old English Bryten(lond), probably also from Latin. The earliest known written references to the British Isles derive from the works of the Greek explorer Pytheas of Massalia; later Greek writers such as Diodorus of Sicily and Strabo who quote Pytheas use of variants such as Prettanikē, "The Britannic [land, island]", and nesoi Brettaniai, "Britannic islands", with "Pretani" being a Celtic word that probably means "the painted ones" or "the tattooed folk", referring to body decoration (see below).

The modern Welsh name for the island is (Ynys) Prydain. This demonstrates that the original Common Brittonic form had initial P- not B- (which would give **Brydain) and -t- not -tt- (else **Prythain). This is best explained as containing a stem *pritu- (Welsh pryd, Old Irish cruith; < Proto-Celtic *kwritu-), meaning "shape, form", combined with an adjectival suffix. This leaves us with *Pritania.

...
Wikipedia