The term "celt" seems to have come about from a copyist's error in many medieval manuscript copies of Job 19:24 in the Latin Vulgate Bible, which became enshrined in the authoritative Sixto-Clementine printed edition of 1592. Where all earlier versions (the Codex Amiatinus, for example) have "vel certe" (the Latin for "but surely"), the Sixto-Clementine has "vel celte." The Hebrew has לעד at this point, which means "forever." The editors of the Oxford English Dictionary "[incline] to the belief that celtis was a phantom word," simply a misspelling of "certe." However, some scholars over the years have treated celtis as a real Latin word.
From the context of Job 19:24 ("Oh, that my words were inscribed with an iron tool on lead, or engraved in rock forever!"), the Latin word "celte" was assumed to be some kind of ancient chisel. Eighteenth-century antiquarians, such as Lorenz Beger , adopted the word for the stone and bronze tools they were finding at prehistoric sites; the OED suggests that a "fancied etymological connexion" with the prehistoric Celts assisted its passage into common use.