Plural of doornail; large decorative nails hammered into doors for strength or ornamentation, famously referenced in the phrase 'dead as a doornail.'
From 'door' + 'nail' (from Old English 'nægel' meaning a fastening pin or spike). Doornails were actual architectural elements, but by the 1500s, the phrase 'dead as a doornail' existed, likely because these nails were hammered so hard they couldn't move.
The phrase 'dead as a doornail' appears in Dickens' A Christmas Carol (1843), but it's at least 300 years older—medieval blacksmiths hammered doornails flat against metal plates, making them literally immovable, which stuck in people's minds as the ultimate image of deadness!
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