The act of masturbating while staying as a guest in someone else's home. Particularly entertaining if it is in the home of someone you do not like.
Harper: "Hey Chuck, I'm going home tomorrow, so you can have the room to yourself."
Chuck: "Thanks man, but I don't need it."
Harper: "You sure? I've been around all week."
Chuck: "I'm good bro, I guesterbated in JJ's room last night while he was asleep."
Harper: "Awww man that's dirty, wish I woulda thought to do that."