My guess would be, Wiktionary is flat-out wrong. The only source for that statement is a century and a half old and, while I don't have access to it to check, the connection between torus and traugus seems tenuous (both phonologically and semantically).
fdb's source, from CNRTL, has a much more plausible explanation (please pardon my translation, I'm very bad at French):
From a Vulgar Latin word traucum attested in the Lex Ripuaria as traugum […] probably originally Gaulish. But the other Celtic languages don't show cognate forms, and it's been proposed that the Gauls borrowed this word from a language spoken in Gaul before their arrival.
French Wiktionary agrees, citing significantly more recent sources.