Literally "blow of mercy", this French phrase is used for any decisive stroke
... A French phrase meaning 
blow of mercy, that is, a final killing blow that puts a fatally wounded person out... [DOC] [TLE] Coup De Grce - Merriam-Webster [PAR] 2 : a decisive finishing blow, act, or event ... the 17th century, "coup de grce" (
literally, a "stroke of grace" or "blow of mercy") originally referred to a ... (In some 
contexts the term is used to refer to the final act of executing a convicted criminal.)