/ ˌkuː də ˈgrɑːs/ noun [ sing. ] (from French , formal )
1.
an action or event that finally ends sth that has been getting weaker or worse :
My disastrous exam results dealt the coup de grâce to my university career.
2.
a hit or shot that finally kills a person or an animal, especially to put an end to their suffering
SYN death blow
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WORD ORIGIN
French , literally stroke of grace .