E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Coup de Grâce.
The finishing stroke. When criminals were tortured by the wheel or otherwise, the executioner gave him a coup de grâce, or blow on the head or breast, to put him out of his misery.
The Turks dealt the coup de grâce to the Eastern empire.Times.
The following is taken from a note (chap. xxx.) of Sir W. Scotts novel The Betrothed
This punishment [being broken on the wheel consists in the executioner, with a bar of iron, breaking the shoulder-bones, arms, thigh-bones, and legstaking alternate sides. The punishment is concluded by a blow across the breast, called the coup de grâce, or blow of mercy, because it removes the sufferer from his agony. Mandrin, the celebrated smuggler, while in the act of being thus tortured, tells us that the sensibility of pain never continues after the nervous system has been shattered by the first blow.