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 (for defs. 1, 2): blackjack
Word origin
[1585–95]
Examples of 'black flag' in a sentence
black flag
Each table had a little red and black flag stuck into a bowl of fruit.
O'Connor, Joe DESPERADOES
A red and black flag was flying from the balcony and on the rooftop an old-fashioned black cannon jutted out in the direction of the city.