Из толкового словаря Вебстера: "Definition of ASS:
1 any of several hardy gregarious African or Asian perissodactyl mammals (genus Equus) smaller than the horse and having long ears; especially : an African mammal (E. asinus) that is the ancestor of the donkey
2 sometimes vulgar : a stupid, obstinate, or perverse person —often compounded with a preceding adjective <don't be a smart-ass>".