Rusty Autoharp means: An act that involves performing an anilingus on a girlfriend while simultaneously reaching out to stimulate her vaginal region by hand. The term “rusty trombone” is derived because women don’t have a trombone like appendage. Furthermore, the original originator maintains that the term sounds funny even though they don’t know how to play an autoharp. (in Community Dictionary, added by Philomelian)