1. [mid-18C+] the buttocks.
2. [20C+] (orig. campus, also rears) a lavatory [the position in the rear of a college or ref. to sense 1].
3. [1930s] (Aus. drugs) a packet containing a measure of a narcotic drug.
see chap someone’s ass under ass n.
see get one’s ass in gear under gear n.
see have someone’s ass under ass n.