A person who faps in the toilet. Most often used to describe a person who habitually masturbates in public restrooms, i.e. a chronic bathsturbater.
Guy 1: "Man, Billy goes to the bathroom in the middle of this lecture every day!"
Guy 2: "Yeah man, didn't you know? Ole Bill is a bathsturbater, and he thinks the prof is sexy."
Girl 1: "Gross!"