1. a pirate flag, usually of black cloth with a white skull and crossbones on it; Jolly Roger.
2. a flag having two yellow and two black squares, signifying the letter L in the International Code of Signals: formerly so called when used by itself to indicate the presence of cholera on board a vessel.
3. a signal given to an automobile racing driver to stop, usually because the car may have a dangerous problem that requires inspection. Also called blackjack(for defs. 1, 2).