A person who does weird things, like stares at you while you sleep, follows you through the club, constant DM's, IM's and likes pictures from 3 years ago, or looks at you for hours through a window. Usually a close friend or relative. you know right away if that person is a
Creepy Dude or does Creepy things. it is not hard to spot the Creepy Dude.
Also see Creeper, and
Charlie Wilson a/k/a Creepy Uncle Charlie