Completely dead or utterly lifeless, with no possibility of revival.
This phrase dates back to the 14th century and refers to the large-headed nails used in medieval doors. These nails were 'clinched' (bent over) on the inside of the door after being hammered through, making them impossible to remove and reuse - hence 'dead' or useless. The phrase gained literary prominence through Shakespeare and Dickens.
The doornail comparison is brilliant because these nails were literally 'killed' by the construction process - once clinched, they could never serve as nails again. It's a surprisingly technical metaphor that most people use without knowing the carpentry knowledge behind it, showing how specialized craft terminology can become universal expressions!
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