A trombone that, when subjected to water or water-based liquids and even air moisture for a long enough time, develops an outer layer of iron oxide most commonly known as rust. It makes a trombone aesthetically and ergonomically unpleasing, as well as difficult to play.
Andy: Hey Bobby, do you want to play my flute?
Bobby: No, I want to play my trombone but it's not working. I dropped it in the water and accidentally left it there for six weeks. Now it is a rusty trombone.
Andy: Well shucks.