What is a rusty trombone?

Rusty Trombone is a euphemism for a sexual act involving a man in a standing position with knees and back slightly bent, with feet at least shoulder width apart in order to expose the anus. The other partner typically is on his or her knees behind the man and performs anilingus while reaching up beneath the testicles or around the body to masturbate the man, 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 penile masturbation; however, other positions and variations are possible.