the final nail in the coffin

US /ðə ˈfaɪnl neɪl ɪn ðə ˈkɔfɪn/
UK /ðə ˈfaɪnl neɪl ɪn ðə ˈkɔfɪn/
"the final nail in the coffin" picture
1.

le coup de grâce, le dernier clou du cercueil

an event that causes the failure of something that had already been weakening

:
The company's latest scandal was the final nail in the coffin for its reputation.
Le dernier scandale de l'entreprise a été le coup de grâce pour sa réputation.
His decision to quit was the final nail in the coffin for the struggling project.
Sa décision de démissionner a été le coup de grâce pour le projet en difficulté.