E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Dead as a door-nail. The door-nail is the plate or knob on which the knocker or hammer strikes. As this nail is knocked on the head several times a day, it cannot be supposed to have much life left in it.
Come thou and thy five men, and if I do not leave you all as dead as a door-nail, I pray God I may never eat grass more.Shakespeare: 2 Henry VI., iv. 10. (Jack Cade.)