rusty knife

when you reach down a girl's pants to give her the ole fingerooski but you accidentally miss and stab her in the poop shoot hence leaving your blade rusty.

A Rusty Jack Knife is a slang term for a geriatric man who porks women much younger than him. The "Rusty Jack Knife" comes in to play as a reference to his penis, which, aged, is rusty (hard to get erect), jacked (may have liver spots or warts), and knifed (smaller than the original youth size.)