drive the final nail in (someone's or something's) coffin

To do something that leads to or brings about one's, someone's or something's ruin, downfall, or failure. The company was already facing a lot of problems. This class action lawsuit is going to drive the final nail in their coffin. Please don't do this. If you reject our loan application, you'll be driving the final nail in our coffin!

Perform some destructive (or self-destructive) act. The term first appeared in print in Peter Pindar’s (John Wolcot’s) Ode no. 15 (1789): “Care to our coffin adds a nail.” It was repeated over the years, and in the early twentieth century was taken up by the Anti-Cigarette League, which announced that every cigarette smoked was a nail in one’s coffin, giving rise to the colloquial name coffin nail for a cigarette. “Have you a coffin nail?” wrote O. Henry (The Higher Abdication, 1907).