A person that hides inside a toilet waiting for a victim, when someone puts his/her butt on the toilet the buttsucker lunges for his/her butt and sucks all the crap out.
You can't leave the toilet seat when your buttsucked, because the power of the buttsuckers sucking is superb.
Can also mean loser.
Rick waited inside the the Mexican restaurant's bathroom's toilet for a victim to buttsuck. Rick is a buttsucker.