A person who creeps forward behind you at a red light, after you pull up a little, even though the light didn't change.
Jon: Hey Mike, pull up a little, let's see if that cutie behind us, bumper humps us.
Jon: Sweet, she did! She's the hottest bumper humper, I've ever been with.
Mike: Here comes the dude behind her, snap, I thnk we started a chain reaction.
a) somebody who will never pass you, but lingers way too close behind you.
b) a tail-gater that fluctuates their speed dramatically on the highway just to piss you off or because they're on their cell phone.
c) a tail-gater who might even pass you, but will eventually slow down and tail-gate someone else.
"Hey dude, check out this guy behind me. He's totally riding my ass!"
"Aw man, I hate that. Pull over and let him pass."
"Naw bro, I did let him pass me earlier, but he slowed down and now he's tail-gating me!"
"Wow! What a bumper humper!"
"Yeah man, and I totally have my cruise control on!"
A bumper humper is a man on man action when he would rather place his penis in other mens asses rather then have sex with a women. This could also be used as a phrase to simply call a man gay.
See that man over there wearing a pink shirt and kissing that other man buddy? He must be a bumper humper.