Green’s Dictionary of Slang

an act of fellatio.

1954		in Partridge DSUE (1984) 443/2: I give you gam for ten bucks.
1960		J. Franklyn Dict. of Rhy. Sl. 108/1: Plate of ham Gam.
1977		D. Powis Signs of Crime 185: Gam Taboo word for fellatio. Never used in mixed company.
1982		P. Bailey Eng. Madam 82: Imagine my surprise when a gentleman insisted that I give him a gam.
1994		I. Welsh ‘A Soft Touch’ in Acid House 50: Gies a barry gam though.
2001		K. Sampson Outlaws (ms.) 123: Puts her arms around us and slides her hand down me trackies and starts giving us a gam.
2015		I. Welsh Decent Ride 160: That big fuckin welt ay yours, Jonty [...] doon her wee throat? Death by gam!
2019		P. Baker Fabulosa 293/1: gam 1. oral sex.