1. A person who is all around DUMB. Everything he/she says or does just doesn't make sense or isn't funny EVER, you can call them a poop cruiser. The ONE person in your crew who EVERYONE picks on is ALSO known as a "poop cruiser".
2. Can also be used to describe a gay guy. (Hence) "Poop" - as in anal penetration. "Cruiser" - As in enjoys the ride of whatever they are 'cruising'.
EX1. Trent said something real retarded at a party and no one thought it was funny. It has been like this for a while, and he is now considered a "Poop Cruiser".
EX2. Cody is a gay kid at school. Everyone knew he was gay and his nickname in school was "Poop Cruiser".