Green’s Dictionary of Slang

1. sexual intercourse.

1896

Farmer Vocabula Amatoria (1966) 117: Exploit, m. The act of kind; ‘tromboning.’.

2. a sex act in which a woman or gay man licks their partner’s anus while simultaneously masturbating him.

1999

Roger’s Profanisaurus in Viz 98 Oct. 28: tromboning v. A thoroughly impractical sex act in which the woman (or gay man) rims a man’s brass eye whilst simultaneously reaching round the front to give it some elbow on his horn, the action being akin to playing a trombone. Also known as a George Jism.