Brythonic (adj.)

c. 1200, "a Celtic native of the British Isles," from Anglo-French Bretun, from Latin Brittonem (nominative Britto, misspelled Brito in MSS) "a member of the tribe of the Britons," from *Britt-os, the Celtic name of the Celtic inhabitants of Britain and southern Scotland before the 5c. Anglo-Saxon invasion drove them into Wales, Cornwall, and a few other corners. In 4c. B.C.E. Greek they are recorded as Prittanoi, which is said to mean "tattooed people."

In Middle English it was exclusively in historical use, or in reference to the inhabitants of Brittany (see Breton); it was revived when James I was proclaimed King of Great Britain in 1604, and made official at the union of England and Scotland in 1707.