black flag
Concise Oxford English Dictionary © 2008 Oxford University Press:
black flag
▶noun
- 1 historical a pirate's ensign, featuring a white skull and crossbones on a black background.
- 2 historical a black flag hoisted outside a prison to announce an execution.
- 3 Motor Racing a black flag used to signal to a driver that he must stop at the pits as a punishment for a misdemeanour.
'black flag' also found in these Oxford entries:

