Those who indulge themselves with Methamphetamine until they become very paranoid and develop a serious case of body tremor.
eg. People who ask for money on a daily basis; people who consistently and rapidly scratch themselves. "Hey check those tweekers arguing with the cops, the one on the left looks all messed up."
eg. Tyrone Biggums from the "Chappelle Show"; Jake Mazursky from "Alpha Dog"; Every character in the movie Spun; That chick from Full House, in real life.