nail in coffin
nail in (someone's or something's) coffin
Something that will eventually contribute to someone 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.
(another) nail in (someone's or something's) coffin
Fig. something that will harm or destroy someone or something. (Alludes to the sealing of a coffin with nails.) Every word of criticism that Bob said about the boss was another nail in his coffin. Losing their biggest customer was the final nail in the company's coffin.