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.)
"God damn! That old guy is a Rusty Jack Knife!"
by The Masticator May 02, 2005