Give no quarter
means to show no mercy, to refuse to spare the lives of opponents.
Pirates played upon fear and death in adopting the black flag as their personal standard. Red, or “bloody," flags were also used to communicate 'No Quarter' to enemies, and were used interchangeably with the black. This not only made pirates instantly recognizable to both their victims and to each other, it also helped instill a sense of fear in all sailors. (To house and feed a prisoner one would give them quarters, or quarter them).