1. [mid-16C+] the buttocks, the posterior.
2. [late 19C+] used anthropomorphically, the back of an object, e.g. a car, a bus.
[late 19C] a large dress-pannier, fashionably affixed to a lady’s dress c.1888.
see ass-kicking n.
[20C+] to be arrogant or to show complete indecision or bewilderment.
[1970s–80s] (US black) to hit or slap someone on the buttocks.