Nail in the Coffin is a popular idiomatic expression, used to refer to something as contributing to the demise of someone or the downfall of something.
It is most predominantly used to label something a grave mistake, that leads to total doom, similar to another phrase, Digging His Own Grave.
The phrase invokes the common imagery of the coffin, which is closely associated with death, with every nail representing a step towards the afterlife.
The origin of “Nail in the Coffin” can be traced back to the work of John Wolcot, published in 1789, under the title Expostulatory Odes to a Great Duke, and a Little Lord By Peter Pindar, Esquire.
In the book, Ode XV features the couplet:
Care to our coffin adds a nail, no doubt;
And ev’ry grin, so merry, draws one out.
The thought was quickly adopted by many at the time, appearing in letters and publications, written in the subsequent decades.
Due to the strong imagery and associations of “Nail in the Coffin”, it became a widely recognized and used expression in the entire Anglosphere, ranging from the US, to the UK and Australia.
The phrase is a veritable commonplace, which can be encountered anywhere, from publications, books, poems, movies, television series as well as music.
One of the most prominent releases of our century, referring to the expression, is the diss track by Eminem, written to Benzino, titled “Nail in the Coffin.”
The idiom, with its versatility and strong imagery, is likely to stay, so you might as well adopt it to your own lexicon, to express yourself more adequately.