As Dead as a Doornail: Origin and Meaning

The meaning is clear. Dead as in "dead as a Dodo", as in not alive or unusable.
Firstly the subtle difference between a nail and a doornail.
A nail is defined as “…a spike or piece of metal with a point and a broadened head so as to be easily driven in by a hammer”. No problem with this, as the nails one finds in any workshop can fit this description.
A doornail has a different description, and runs like this “…a large headed nail with which doors were formally studded” (Note…. past tense)
(Both definitions from Oxford dictionary).
Uses of iron nails goes back to ancient Roman times, and were made by hand. A labor intensive exercise that made them scarce and expensive.
During the 18th centuary mechanical nail manufacture took place and the occurrence of hand made nails decreased significantly.
During the middle ages nails were recycled whenever possible, which makes sense when one thinks of the effort that went into making them.
The problem was with doors the nails were knocked right through, and the part sticking out at the back was hammered back flush with the door. These were most probably bigger than normal nails as they were studded for decorative purposes. The reason for bending the nails back, (called clinching or clenching) I could not find, other than a reference that stated this made the door stronger. Doors were made of two pieces that were nailed together.
These nails could not be recycled and were useless for any further use. Therefore dead as a doornail in carpentry terms..
This expression was in print during the 1300’s and Shakespeare used similar words in Henry VI.
Reference was also found to a “doornail” as a flat piece of metal fixed to the door that one banged the doorknocker on, to get a nice sound so that people would know that there was someone at the door. The dead part was the dull sound the knocker made.
Other references state that the nail on the doorknocker had been hit so many times against the door it must be dead.
Dead as a Dodo has a similar meaning.
Image from Wikipedia