This term is used in such expressions as “to give no quarter” or “cry quarter”; in regard to hostilities by land, sea or air, denial of quarter means refusing to spare the life of anybody, even of persons manifestly unable to defend themselves or who clearly express their intention to surrender. 
International humanitarian law prohibits the use of this procedure, that is, ordering that there shall be no survivors, threatening the adversary therewith, or conducting hostilities on this basis. 
Denial of quarter is a grave breach of international humanitarian law.  



giving or ordering no quarter

PI, 40

CIHL, 46