Green’s Dictionary of Slang

1. to rack the slide of a shotgun, loading a cartridge into the breech.

1990

S. Morgan Homeboy 78: Rooski [...] tromboned the shell home with a clash.

2. to perform a sex act in which a woman or gay man licks their partner’s anus while simultaneously masturbating him; thus tromboner n.

2000		Indep. Rev. 28 Mar. 1: De only fing me dream about iz bonin Mariah Carey while havin Jennifer Lopez trombone me from behind.
2004		Guardian G2 27 Sept. 16/3: His band of ‘frottagists’, ‘Roman-shower enthusiasts’, ‘Funchists’, ‘tromboners’ and ‘upper-deckers.’.