ass 1 
(ăs)
(ăs)n. pl. ass·es (ăsĭz)
1. Any of several hoofed mammals of the genus Equus, resembling and closely related to the horses but having a smaller build and longer ears, and including the domesticated donkey.
2. A foolish or stupid person.
[Middle English asse, from Old English assa, perhaps a hypocoristic for Old English dialectal (Northumbrian) asal, ultimately (perhaps via a Celtic form akin to Old Irish asal) from Latin asinus; see ASININE.]