One who informs upon associates; one who withholds portions of loot, with intent to defraud associates; one who leaves associates without warning or means of escape when interrupted in crime; a traitor. "That kid brother of mine turned out to be a dumper. He spilled his guts (informed) on Icecart and he got me loused up (under suspicion) with some good people (underworld's best citizens)."

- american underworld dictionary - 1950