"Show no quarter" is an idiomatic expression that means to not show mercy or leniency towards someone. Some synonyms for this phrase include "give no mercy," "be ruthless," "show no compassion," "be merciless," and "be pitiless." These phrases convey a similar message to "show no quarter" and are often used in situations where a tough stance is needed. Other phrases that convey a similar sentiment include "go for the kill," "show no empathy," and "be indifferent." Whether used in literature, lyrics, or in casual speech, the use of these phrases shows a willingness to be ruthless towards someone or something.