The act of invading a friend's pool, and going for a swim without its owner's knowledge. The friend can either be asleep, in a windowless room, away from the house. The friend can't have given any prior permission to enter the pool: It's actually a bonus to break the rules in order to therylflyze.
John: I totally therylflyzed at jenn's house last night... she has no idea.
Sally: Sweet, I'm coming next time.
John: Don't worry about it.