Green’s Dictionary of Slang

moon v.

[moon n. (1a)]

1. (US) to have anal intercourse with.

[US]A. Berkman Prison Memoirs of an Anarchist (1926) 169: ‘You know what “moon” is, don’t you?’ he asks, abruptly, with an evident effort to suppress a smile. [...] ‘Moonology, my Marktwainian Innocent, is the truly Christian science of loving your neighbor, provided he be a nice little boy.’.
[US](con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 71: Danny O’Neill [...] said he’d like to start mooning punks.

2. (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.

[US]Current Sl. II:3 4: Moon, v. To expose gluteus maximus in public. (possibly from Hawaii, Okole maluna, ‘bottoms up.’).
[US](con. 1950s) H. Junker ‘The Fifties’ in Eisen Age of Rock 2 (1970) 101: Certain college studs [...] displayed their naked asses to passersby, an act variously called [...] mooning, handing out the b.a., gotcha.
[US]Sat. Rev. (US) 15 July 64: One boy rhapsodizes about his success at ‘mooning,’ a boyish trick of sticking one’s bare behind out the window at passing girls.
[US]C. Hiaasen Double Whammy (1990) 23: The cameraman dreamed of the day when Dickie Lockhart would [...] drop his drawers to moon his little ole fishing pals across America.
[UK]Guardian G2 28 July 3: Newspapers published pictures of William’s friends mooning from a black taxi.
[US]S. King Dreamcatcher 603: It was called ‘mooning the assholes’ and he wanted to do it.
[UK]D. O’Donnell Locked Ward (2013) 159: She could see the [...] bitch at the picture windows. So she flicked a double set of V’s at her, mooned and then made good her flight.
[Aus]N. Cummins Tales of the Honey Badger [ebook] One clown dropped his dacks and mooned the crowd.

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

[UK]P. Baker Blood Posse 303: ‘Drop your pants and spread your cheeks.’ [...] ‘I ain’t mooning for no one.’.