What is "rusty trombone"

n. (context vulgar English) The act of performing anilingus whilst reaching in between the legs to administer manual#Etymology_2 petting#Noun of the penis.

Rusty trombone as a sexual act in which a man stands with his knees and back slightly torqued with feet at least shoulder width apart to expose his anus. The other partner typically kneels behind the man and performs anilingus while reaching up beneath the testicles or around the body to manually administer rapid up and down motions of the penis, mimicking the motions of a trombone player. The act is defined primarily by the physical orientation of the partners and the combination of anilingus with manual sexual stimulation; however, other positions and variations are possible.