English abounds with similes in the form of ‘as x as a y‘ but where did this one come from?
The answer is not clear, but the expression has been around since before Shakespeare (and the good Bard used it in Henry IV).
Quite possibly ‘as dead as a doornail’ has survived down the centuries because it sounds so good – as dead as a nail is not quite as pleasing to say!
It is thought that the doornail in question could have been the large one under the knocker of a medieval door. This nail was hit on the head so many times it was definitely dead. Also, in the world of carpentry, a nail that is hammered all the way through and then has the protruding part hammered back up for strength (a process called clinching) is no longer useable. It is dead.
Now as dead as a dodo is easier to understand. Before this poor creature became extinct the phrase ‘as rare as a dodo’ was around for a while.