Green’s Dictionary of Slang

moon v.

[moon n. (1a)]

1. [late 19C–1930s] (US) to have anal intercourse with.

2. [1960s+] (orig. US) to drop one’s trousers and underpants and present one’s bare buttocks to onlookers, often performed through a car window; thus mooning n.

3. [1990s+] in weakened form of sense 2 above, to bare one’s buttocks.