Wiktionary
dead as a doorknob
a. (context simile English) entirely, unquestionably or certainly dead.
Usage examples of "dead as a doorknob".
It would have been a pleasant room if it wasn't for the certain knowledge that Uncle Harry or Aunt Minnie or Morty the mailman was naked in another part of the house, dead as a doorknob, getting pumped full of formaldehyde.
I went out with him a couple times, and that thing of his was dead as a doorknob.
He was just lyin dead on the fifth floor, dead as a doorknob, face all contorted like hed seen a ghost.