Green’s Dictionary of Slang

(also bum off) to sodomize, thus n. bumming, anal intercourse, inter-crural intercourse.

1909		Farmer & Henley Sl. & Its Analogues (rev. edn) 425/2: bum, verb (venery).—..(b.) To bugger.
1968		R. Lambert Hothouse Society 273: [teenage speaker] ‘The Housemaster has been trying to bum a certain member of the house. He tries to get the Corps members to take off their uniforms...’ [Simes:DLSS].
1970		A. Sillitoe Start in Life (1979) 347: If he pansies after a young man he’s buggering his son [...] If he gets off with an older man he’s being bummed by his father.
1971		G. Oakley Call Girl 71: We have to watch and to take part in all sorts of perversions, from whipping to bumming.
1983		J. Robinson Teardrops on My Drum 107: ‘I suppose you want to bum the pair of us now’ [Simes:DLSS].
1986		Graffito (Melbourne) There are two guys on the picture and one guy’s bumming the other guy off [Simes:DLSS].
1986		G. Wotherspoon Being Different 152: [H]e’d shown Alec a new game, that of ‘bumming’. Stretched out on his belly Alec allowed this young man to place his penis between his legs and writhe about on his back [Simes:DLSS].
1999		(con. 1979–80) A. Wheatle Brixton Rock (2004) 33: We used to say some housefather was bumming him.
2000		M. Manning Get Your Cock Out 57: When I was seven the fucking idiot started using my arse like a dartboard, bumming me stupid every fucking night.
2004		P. Howard PS, I Scored the Bridesmaids 158: I said, ‘Hello,’ not, ‘I wanna bum you.’.
2011		Internat. Independent 19 July 37/3: I fantasised about Piers being bummed in prison.
2016		Times Online 15 Apr. 🌐 CW was told so much amazing gossip that [...] it actually compiled a Rolodex of gossip in which ‘B’ — for ‘bumming’ — was 38 entries long.