bumper humper

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!"