mutton

From Middle English motoun/moton, borrowed from Old French mouton ("sheep"), which came from Vulgar Latin moltō, itself from Gaulish *multon-, tracing back to Proto-Celtic *moltos, meaning "ram" or "wether."

The word simply moved language to language, carrying the sense of a sheep or its meat along with it, until English settled on it for the cooked flesh specifically.

It's a doublet of mouton, the French word for sheep that English also borrowed directly, so the two are essentially twins from the same root.