Deader Than a Door Nail
There are two schools of thought on this one…
1) A door nailed shut isn’t really a door anymore – so a “door nail” is an oxymoron – it doesn’t exists. Nothing deader than something that doesn’t exist.
2) Back in the day, the door bell was a “door knocker” that you rapped against “nail” driven in the door. With all the constant rapping, the nail would surely be dead.