Etymology
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welch (v.)
1857, racing slang, "to refuse or avoid payment of money laid as a bet," probably a disparaging use of the national name Welsh. Related: Welched; welching.
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Welsh (adj.)
Old English Wielisc, Wylisc (West Saxon), Welisc, Wælisc (Anglian and Kentish) "foreign; British (not Anglo-Saxon), Welsh; not free, servile," from Wealh, Walh "Celt, Briton, Welshman, non-Germanic foreigner;" in Tolkien's definition, "common Gmc. name for a man of what we should call Celtic speech," but also applied in Germanic languages to speakers of Latin, hence Old High German Walh, Walah "Celt, Roman, Gaulish," and Old Norse Val-land "France," Valir "Gauls, non-Germanic inhabitants of France" (Danish vælsk "Italian, French, southern"); from Proto-Germanic *Walkhiskaz, from a Celtic tribal name represented by Latin Volcæ (Caesar) "ancient Celtic tribe in southern Gaul."

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).
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which (pron.)
Old English hwilc (West Saxon, Anglian), hwælc (Northumbrian) "which," short for hwi-lic "of what form," from Proto-Germanic *hwa-lik- (source also of Old Saxon hwilik, Old Norse hvelikr, Swedish vilken, Old Frisian hwelik, Middle Dutch wilk, Dutch welk, Old High German hwelich, German welch, Gothic hvileiks "which"), from *hwi- "who" (from PIE root *kwo-, stem of relative and interrogative pronouns) + *likan "body, form" (source also of Old English lic "body;" see like (adj.)). In Middle English used as a relative pronoun where Modern English would use who, as still in the Lord's Prayer. Old English also had parallel forms hwelc and hwylc, which disappeared 15c.
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