Any of several species of horse-like animals, especially Equus asinus, the domesticated of which are used as beasts of burden.

A stupid person.

"That new kid left the cap off the syrup bottle again! What an ass."

Buttocks.

Sex.

"I’m going to go down to the bar and try to get me some ass."

Anus.

Used in similes to express something bad or unpleasant.

Used after an adjective to indicate extremes or excessiveness.

"That was one big-ass fish!"

(metonymy) One's self or person, chiefly their body.

"Get your lazy ass out of bed!"