another nail in the coffin

another nail in the coffin (of) AND the final nail in the coffin (of) (an event which causes the failure of something that had already started to fail) — еще одна капля; последняя капля; | добить (разг.)

Example: I think that argument was the final nail in the coffin of our friendship. (CIDI) — Думаю, что та ссора стала последней каплей — нашей дружбе пришел конец.

see also
[straw that breaks the camel's back]
[last straw]