Definition: An event that causes the failure of something that had already started to fail.Origin: Many coffins were plain, handmade wooden (often pine) boxes whose lids were nailed shut for burial. This gave rise to the idioms "another nail in the coffin" and "final (or last) nail in the coffin." Both refer to any event that contributes to ("another nail") or causes the failure ("final nail") of something that was already doomed or had already begun to fail. It's the most recent in a series of events leading to a seemingly unavoidable negative outcome.