take no prisoners. literally it means to kill all of your captured enemy, figuratively it means to show no mercy
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Uses:
How the scene comes back to me! I can hear him now, with that war-note in his voice, flaying them with his facts, each fact a lash that stung and stung again. And he was merciless. He took no quarter, and gave none.
Jack London. The Iron Heel
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However, now that the glove had been thrown down to him, he had boldly picked it up and demanded the appointment of a special commission to investigate and verify the working of the Board of Irrigation of the lands in the Zaraisky province. But in compensation he gave no quarter to the enemy either.
Leo Tolstoy. Anna Karenina
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This room is one of the most dangerous places I could be. If I were caught there would be no quarter, but I’m beyond caring.
Margaret Atwood. The Handmaid's Tale (1986)
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it comes to the soldier, war-mad on a stricken field and refusing quarter; and it came to Buck, leading the pack, sounding the old wolf-cry, straining after the food that was alive and that fled swiftly before him through the moonlight.