What does a rusty trombone mean?

"Rusty trombone" is a slang term used to refer to anal sex. It is a combination of the words "rusty" (referring to the potential discomfort or pain associated with anal sex) and "trombone" (a musical instrument that is long and cylindrical, similar to the shape of a penis). The term is often used in a humorous or lighthearted way, but can also be used in a more serious or clinical context.