Nail in the coffin

It was just another nail in the coffin. That was the nail in the coffin. It was kind of a nail in the coffin.

In the old days, coffins were just wooden boxes, which were nailed shut just before burial. So when someone says that something is a "nail in the coffin", that person usually means that it really finalizes something. Usually, it's something negative, like a loss. If a football (American football, not soccer) team is already up by ten points in the fourth quarter, another touchdown for that team might really put the nail in the coffin. They were already winning, and it was already clear that they would probably win, but now it is that much more clear. It might end any realistic chance of the other team winning. So it is a nail in the coffin of the other team's winning chances.

It was just another nail in the coffin. That was the nail in the coffin. It was kind of a nail in the coffin.

This idiom should probably not be used to describe situations that actually relate to death, injury, or severe illness. It may offend someone. It would also be an unusually strong and perhaps offensive way to talk to a person about their own losses or your own victories.