The dog descriptor mutt meaning a non-purebred is first seen in dog breeding magazines in 1910. The adjective was actually first applied to people, with a meaning of “idiot”, a few years earlier in 1901. It was a short form of muttonhead, which had been a common word for “idiot” since 1803.

Mutton itself is one of the Norman French words that were borrowed into English after the Norman Conquest. Norman French was the high class language to the peasants’ English. One of the ways this manifested is the French words for animals were used for meat, which nobles discussed, while the English words remained in use for animals. Some words for meat with this origin borrowed between 1066 and 1300 are mutton, beef, pork, poultry, veal, venison, and uh, pigeon. The corresponding terms in Old English were scēap, cū, swīn, cicen, cealf, heorot (Modern English hart; deer meant “animal”, as with German Tier), and dūfe (Modern English dove).