nail in one's coffin

Something that will eventually contribute to someone's or something's downfall or death. That class action lawsuit is going to be the nail in the company's coffin. Every beer you drink is a nail in your coffin.

Something that might hasten or contribute to one's death, as in Every cigarette you smoke is another nail in your coffin. This expression, alluding to fastening down a coffin lid, is today almost always used for a harmful habit such as tobacco use (giving rise to the slang term coffin nail for a cigar or cigarette). The idea was first expressed in an ode by Pindar (the pseudonym of John Wolcot) in 1792: "Care to our coffin adds a nail, no doubt."