coup de grâce
noun [ S ] formal uk/ˌkuː də ˈɡrɑːs/ us/ˌkuː də ˈɡrɑːs/
致命一击;（为免除人或动物的痛苦而给予的）慈悲一击
an action that ends something that has been gradually getting worse, or that kills a person or animal in order to end their suffering
Jane's affair was the coup de grâce to her disintegrating marriage. 简的婚外情终结了她本已破碎的婚姻。