The French expression coup de grâce (also written coup de grace) literally means "blow of mercy", or death-blow. It refers to the mercy killing intended to end the suffering of a mortally wounded creature. More figuratively, it can also refer to a cataclysmic event or decisive act that brings about drastic change, for example:
The coup de grace was the closing of the Shell refinery in 1985.[1] (http://www.lonelyplanet.com/destinations/caribbean/curacao/history.htm)
The French pronunciation of coup de grâce is [kud ɡʁas], (2 syllables, not 3) which is not obvious to many English speakers. Some English speakers may pronounce it instead as [ku də ɡɹa], which sounds like coup de gras ("blow of fat"). In French the plural of coup de grâce is coups de grâce ("blows of mercy"), which is pronounced the same as the singular form. The English language is a West Germanic language that originates in England. ...
The practice of the coup de grâce can be traced back to the beginning of recorded history. It has generally been somewhat less controversial than euthanasia, since it specifically applies only to the killing of the mortally wounded. Euthanasia (Greek, good death) is the practice of killing a person or animal, in a painless or minimally painful way, for merciful reasons, usually to end their suffering. ...