1. A male, often a slight odd-ball or creep, who has an insatiable lust for the female bottom which goes way past perverted and borders on psychotic. Often lurks in bushes or back-alleys and pounces when least expected to grab that ass!
2. A man who enjoys simulating the "bumming" action behind a woman when she has bent over for some reason.
Dale: Umm, excuse me Janine, you seem to have dropped your purse.
Janine: Oh, so I have, your such a nice guy Dale (proceeds to bend down and retrieve it)
Dale: Giggidy (performs the bumming motion)
Gary: You are such a bum fiend...