Dead as a doornail

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.