A rustry trombone is when a kinky german chick eats your asshole while giving you a handjob, thus going through the motions of playing the trombone. Typically, the guy is standing up and the girl is kneeling
"tossin the salad while yankin the tubesteak smothered in underwear. unless there is some wanted or unwanted gas. then it's reffered to as a rusty gillespie."
In addition to the above statement, there is also the secondary humor involved with the muffled low end sound that would actually come out of a rusty trombine.
sheryl found that the old rusty trombone could play a perfect b flat