Originally termed in the movie "South Park: Bigger, Longer, and Uncut." Commonly mis-spelled as ass beehonker, this term refers to anal intercourse between two men. Splunking itself is decending into a deep cave, which is fitting.
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.)