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.)
Falstaff. What! is the old king dead? Pistol. As nail in door.
Shakespeare: 2 Henry IV., v. 3.
He is dead. “Gone to the world of light.” “Joined the majority.”
Let the dead bury the dead. Let bygones be bygones. Don't rake up old and dead grievances.
“Let me entreat you to let the dead bury the dead, to cast behind you every recollection of bygone evils, and to cherish, to love,to sustain one another through all the vicissitudes of human affairs in the times that are to come.” —Gladstone: Home Rule Bill (February 13th, 1893).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894