Idiom: (a) nail in the coffin / or a nail in someone/something’s coffin
A coffin (see picture) is a long, narrow box in which a corpse is buried. A coffin is often made of wood.
To close a wooden coffin, nails (small metal spikes) are usually hammered into it. So the final or last nail used would completely close the coffin. At that point, you're dead, cannot get out and are ready to be buried in the ground. This is obviously a very final state to be in.
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