carcass: [14] English first acquired this word from Anglo-Norman carcois, and early forms were carcays and carcoys. Spellings similar to modern English carcass begin to emerge in the 16th century, and may be due to reborrowing from French carcasse, to association with the noun case ‘container’, which meant ‘body’ in the 16th century, or to a combination of both. The usual current spelling throughout the English-speaking world is carcass, but British English also uses carcase. The word’s ultimate origin is unknown.
carcass (n.)
late 13c., from Anglo-French carcois, from or influenced by Old French charcois (Modern French carcasse) "trunk of a body, chest, carcass," and Anglo-Latin carcosium "dead body," all of uncertain origin. Not used of humans after c. 1750, except contemptuously. Italian carcassa probably is a French loan word.