ass

The word 'ass' for the animal has a long and winding journey into English. It entered Old English from Old Irish, which borrowed it from the Latin 'asinus', a term of likely pre-Latin Mediterranean origin. The meaning evolved from the literal animal to a term for a foolish person by the early 15th century, drawing on the donkey's stereotypical stubbornness and stupidity. The vulgar slang for buttocks emerged in the mid-18th century, primarily in North America, likely via a metonymic shift from 'arse', the older term for buttocks, influenced by phonetic similarity and euphemistic avoidance.

This is a simplex word of Germanic origin. The Old English form 'assa' was borrowed from Old Irish 'assan', which in turn came from Latin 'asinus', meaning 'donkey'. Its functional role is as a complete lexical root.