A person who steals cigarette butts out of public ashtrays and smokes them. They are usually very sneaky and attempt to do it when no one is watching to avoid the embarrassment of being caught doing something so profoundly disgusting. Butt bandits are usually the very poor and the underage.
I remember I left a half smoked cigarette in the ashtray outside of work on my last break and now it's gone. It must have been stolen by a butt bandit.