One who's conception is the result of an accidental leakage and transfer of semen from the soon-to-be mother's anus to her vagina. Assuming the feces covered sperm reach an egg cell and successfully fertilize said cell, the outcome will be a rather poor excuse for a human child. This child (Butt Troll) will be forever ridden with terrible hygiene and an uncommonly low level of intelligence.
Brittany's parents should have paid a little more attention to their post-anal-sex clean up process. That girl is smelling up this entire building. I guess I wouldn't expect much else from a Butt Troll though.