black flag 
noun
1 historical A pirate’s ensign, typically thought to feature a white skull and crossbones on a black background.She is known throughout the Royal Navy as the ‘Black Duke’ and she flies a black flag from her mainmast at all times....
- Could she set sail again and raise her black flag, if only in our dreams?
- The Pacific Northwest is likewise out - too much rain and too many cloudy days make me hoist the black flag and start looking for throats to cut.
2 historical A black flag hoisted outside a prison to announce an execution.
3 Motorsports A black flag used to signal to a driver to stop at the pits as a punishment for a misdemeanour.It was a relief that there were no penalty points for bad driving as I picked up three black flags - meaning a stop and go penalty in the pit lane - in as many laps....
- Montoya is disqualified from the race after being given a black flag for a technical infringement relating to the incident in the start where he used the spare car.
- Of course, I didn't want to pass him and get a black flag.