
Coup de grâce (French)
The crowning blow / The final blow / A nail in someone's coffin / The last straw / The final nail in someone's coffin

Colpo di grazia 
The crowning blow / The final blow / A nail in someone's coffin / The last straw / The final nail in someone's coffin

Colpo di grazia 
Meanings
Lit.: a death blow administered to kill to end the suffering of a mortally wounded person or animal. Fig.: an action that tops an event that has been gradually getting worse
Examples
Their marriage was already on the rocks and discovering her husband's years-long affair was the coup de grâce
Il loro matrimonio era già in crisi, ma la scoperta della relazione adulterina del marito gli ha dato il colpo di grazia
I was already dead tired, and that dinner date was the crowning blow
Ero già stanco morto. Quell'invito a cena mi ha dato il colpo di grazia
The rise in property taxes came as the coup de grâce; already deep in debt, he was now forced to sell the business
L'aumento delle tasse sulla proprietà gli ha dato il colpo di grazia: già sommerso dai debiti, ha dovuto svendere la sua attività Origin
French for "stroke of mercy", the phrase originally referred to the merciful fatal stroke that put a dying man out of his misery