The Mabinogeon by Thomas Bulfinch

The earliest inhabitants of Britain are supposed to have been a branch of that great family known in history by the designation of Celts. Cambria, which is a frequent name for Wales, is thought to be derived from Cymri, the name which the Welsh traditions apply to an immigrant people who entered the island from the adjacent continent. This name is thought to be identical with those of Cimmerians and Cimbri, under which the Greek and Roman historians describe a barbarous people, who spread themselves from the north of the Euxine over the whole of Northwestern Europe.

The origin of the names Wales and Welsh has been much canvassed. Some writers make them a derivation from Gael or Gaul, which names are said to signify "woodlanders; " others observe that Walsh, in the northern languages, signifies a "stranger," and that the aboriginal Britons were so called by those who at a later era invaded the island and possessed the greater part of it, the Saxons and Angles.