Many other nouns can be substituted for "doorknob", perhaps the oldest being "doornail" from William Shakespeare, in Henry VI, Part 2, act 4, sc. 10, l. 40-1. Jack Cade. Also in A Christmas Carol. Other common forms include doormat, dodo, rock, graveyard.
Possibly from a misinterpretation or misunderstanding of the expression dead as a doornail. .