The English version of Wiktionary's page on 'trou' (French for 'hole') avers that it's:

From Medieval Latin traugus, a "barbarous" Latin word first attested in the Ripuarian Law, probably related to torus (“round hill”).[1]

Related to Catalan trauc, Occitan trauc.

That Wiktionary page for 'torus' lists its most general meaning as:

  1. a swelling, protuberance, bulge, knot

But the French version avers a different etymology, one that's pre-Celtic.

  • The word torus has a number of meanings as listed in Wiktionary and many other dictionaries. Why do you think it should be construed as "round hill" specifically? The source Wiktionary quotes for this choice of translation is a book from 1844. // If your goal is to understand the etymology of a French word, I think asking at the French site is better. If the question is focused on Latin, can you elaborate on the Latin side of things? – Joonas Ilmavirta May 26 '18 at 23:09
  • @JoonasIlmavirta I'm interested about the Latin side, yes. You're correct that I shouldn't fixate on 'round hill'. I replaced it with the more general meaning. – AYX.CLDR May 27 '18 at 3:51

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.

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