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.
by bobbo421 October 28, 2008
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.)
by The Masticator May 3, 2005