The term “dead as a doornail” was first seen in print in 1350, but was, perhaps, popularized by William Shakespeare in his play King Henry VI in 1590:

Brave thee! ay, by the best blood that ever was
broached, and beard thee too. Look on me well: I
have eat no meat these five days; yet, come thou and
thy five men, and if I do not leave you all as dead
as a doornail, I pray God I may never eat grass more.

But why compare death to a doornail. Doornails are large nails. The nails were hammered through and then the protruding end was bent over to secure it. After a doornail was used once, it could not be taken and used due to the bending of the nail. Thus, a doornail would be dead (or not usable) a second time.