1690s, from French coup de grâce, literally "stroke of grace;" the merciful death-blow that ends another's suffering (see coup).
The final blow: “He had been getting deeper and deeper in debt; the fates delivered the coup de grâce when he died.” The phrase is French for “stroke of mercy.” It originally referred to the merciful stroke that put a fatally wounded person out of his misery or to the shot delivered to the head of a prisoner after he had faced a firing squad.