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).
1785, an absurd perversion of (Welsh) rabbit, as if from rare (adj.) + bit (n.). See Welsh.
1520s, of a people of what is now souther and southeastern Belgium, also of their language, from French Wallon, literally "foreigner," of Germanic origin (compare Old High German walh "foreigner"). The people are of Gaulish origin and speak a French dialect. The name is a form of the common appellation of Germanic peoples to Romanic-speaking neighbors. See Vlach, also Welsh. As a noun from 1560s; as a language name from 1640s.
Old English walhnutu "nut of the walnut tree," literally "foreign nut," from wealh "foreign" (see Welsh) + hnutu (see nut). Compare Old Norse valhnot, Middle Low German walnut, Middle Dutch walnote, Dutch walnoot, German Walnuss. So called because it was introduced from Gaul and Italy, distinguishing it from the native hazel nut. Compare the Late Latin name for it, nux Gallica, literally "Gaulish nut." Applied to the tree itself from 1600 (earlier walnut tree, c. 1400).
"to cheat, to drive a hard bargain," 1824, from Jew (n.) (compare gyp, welsh, etc.). "Though now commonly employed without direct reference to the Jews as a race, it is regarded by them as offensive and opprobrious" [Century Dictionary, 1902]. The campaign to eliminate it in early 20c. was so successful that people also began to avoid the noun and adjective, using Hebrew instead.
Now I'll say 'a Jew' and just the word Jew sounds like a dirty word and people don't know whether to laugh or not. [Lenny Bruce, "How to Talk Dirty and Influence People," 1965]