Wiktionary
n. (context idiomatic English) Something that is ultimately responsible for a previously prospective demise or failure.
Usage examples of "final nail in the coffin".
Sir Robert's interest in Satanism and the occult shortly before his death was only the final nail in the coffin.
A final nail in the coffin of SF was the uncontrolled meltdown at Three Mile Island in 1979.
As if seeing what was below them would make it real and put the final nail in the coffin.
The odds against such a perfect matching of dates could be calculated only in astronomical terms: while the complete disappearance of the crews of four small boats that had come to grief in so very limited an area was the final nail in the coffin of coincidence.
But that last visit to the old house, that whole scene with Megan and her new boyfriend, that had pretty much put the final nail in the coffin, hadn't it?