You may be more familiar with 'coup d'état',(coo de tah) the violent and forceful removal of an existing government (via revolt). Coup de grâce (coo de grah) takes the same idea of a coup (a decisive finishing blow/act) and makes it an act of mercy....literally translated, a stroke of grace. That means that a coup de grâce is a deathblow or deathshot administered to end the suffering of one mortally wounded.
In a more modern sense, coup de grâce has also come to be used figuratively to refer to the culmination of a bad or deteriorating situation. For instance, "the pandemic was a coup de grâce for _______." Fill in the blank with your choice, really...
Spicy literary question: Were George's actions at the end of Of Mice and Men a coup de grâce or was he acting from a place of abled/'disabled' superiority? or does focusing on Lennie's limits detract from the emotional burden George has to deal with? Was it mercy or was it self-interest? Are those things mutually exclusive?
Let me know
–C
