Old English Wilisc, Wylisc (West Saxon), Welisc, Wælisc (Anglian and Kentish), 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 to speakers of Latin, hence Old High German Walh, Walah "Celt, Roman, Gaulish," and Old Norse Valir "Gauls, Frenchmen" (Danish vælsk "Italian, French, southern"); from Proto-Germanic *Walkhiskaz, from a Celtic name represented by Latin Volcæ (Caesar) "ancient Celtic tribe in southern Gaul." 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, hence Welsh rabbit (1725), also perverted by folk-etymology as Welsh rarebit (1785).