The phrase "give quarter" means to show mercy or withhold from attacking enemies in warfare. It is an idiom associated with compassion and forgiveness. The antonyms, therefore, are words that convey the opposite meaning of "give quarter." They are "show no mercy," "be cruel," "attack without respite," and "fight to the end." These words tend to describe a more aggressive and confrontational attitude towards the enemy. In modern parlance, the phrase is also used to describe someone who refuses to give up or surrender. In such situations, the antonyms would imply giving in or accepting defeat, rather than continuing to persevere.