A large nail used on doors (now archaic); or a dead or lifeless thing, as in the phrase 'dead as a doornail.'
From 'door' and 'nail.' In medieval times, large nails were driven through door knockers and hinges. The phrase 'dead as a doornail' emerged because these nails were hammered flat and could never move again.
Nobody uses doornails anymore, but 'dead as a doornail' is STILL a phrase because once that nail was hammered flat, it was truly and completely finished—a perfect metaphor for absolute deadness!
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