The act of using your vehicle as support (and/or to block your bare ass from view) when you can't make it to a "real" restroom and instead have to use the side of the road as a toilet. Can be the result of either an emergency or laziness and is preferably done in a secluded place.
Earlene drank her Boone's wine so fast on the way to the party that we had to stop and let her bumper-pee at least three times before we even got there.
I can't hold it to the next rest stop -- you're gonna have to stop and let me bumper-pee or I'm gonna wet my pants in your car!!!
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.
Mike: Okay...
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.
Annoying and often Idiotic slogans, sayings and proclamations that are attached to the bumpers of the vehicles of usually rednecks or aging hippies.
These abrasive slogans are displayed by the proud owners in lieu of actual original thought and expression. Ditto for the stupid fishies that fundies adorn their autos with, smugly trying to project some sort of superiority, but only showing their boundless stupidity.