From Old French 'mouton' (sheep), from Gaulish Celtic *multo- (ram) — uniquely the only Norman meat-word with Celtic roots.
From Anglo-Norman 'moton,' from Old French 'moton' (modern French 'mouton,' meaning sheep), from Medieval Latin 'multō' (accusative 'multōnem'), probably from Gaulish *multo- (ram, male sheep), of Celtic origin. Unlike the other Norman meat-words (beef, pork, veal), 'mutton' has a Celtic rather than classical Latin origin, reflecting the importance of sheep-herding in pre-Roman Gaul. It completes the famous quartet of French meat-words paired with English animal
Unlike beef (Latin), pork (Latin), and veal (Latin), 'mutton' traces not to classical Latin but to Gaulish Celtic — the language of pre-Roman France. The Gauls were renowned sheep-herders, and their word for ram (*multo-) survived the Roman conquest of Gaul, passed into French, and then crossed the Channel with the Normans in 1066. It is a Celtic word that conquered England disguised as French.