dead as a doorknob

Possibly from a misinterpretation or misunderstanding of the expression dead as a doornail. See usage notes below.

dead as a doorknob

1. (simile) Entirely, unquestionably or certainly dead.

   I tried the flashlight, but the battery was as dead as a doorknob.

   I found the mouse who lived in our wall, lying on his back with his feet in the air—as dead as a doorknob.

Many other nouns can be substituted for "doorknob", perhaps the oldest being "doornail" used by William Shakespeare, in Henry VI, Part 2, act 4, scene 10; and also by Charles Dickens in A Christmas Carol. Other common forms include doormat, dodo, rock, graveyard.