1841, "pertaining to the uppermost strata of the Paleozoic era," named by British geologist Sir Roderick Impey Murchison (1792-1871) for the region of Perm in northwestern Russia, where Murchison had studied rocks from this epoch. His original definition of the rock system was somewhat broader in each direction, chronologically, than the current one.
As a noun, "the Britons," also "the Welsh language," both from Old English. The word survives in Wales, Cornwall, Walloon, walnut, and in surnames Walsh and Wallace. Borrowed in Old Church Slavonic as vlachu, and applied to the Rumanians, hence Wallachia. Among the English, Welsh was used disparagingly of inferior or substitute things (such as Welsh cricket "louse" (1590s); Welsh comb "thumb and four fingers" (1796), and compare welch (v.)). Welsh rabbit is from 1725, also perverted by folk-etymology as Welsh rarebit (1785).