A "Jack Dempsey" refers to one beating his meat very aggressively, as the legendary boxer Jack Dempsey flogged his opponents.
Dude, I got so excited over that midget porn I was watching that I stole some face cream from my sister's make-up bag and gave myself a "Jack Dempsey". Now my pecker is sore.
A "Jack Kerouac" refers to beating off while "On the Road". This is in tribute to the famous author and his most well know story.
Man! My cross country trip was great! Anytime I got bored, I just whipped my dong out and gave myself a little "Jack Kerouac". I sure got some funny looks from people driving by, but it was worth it.