Green’s Dictionary of Slang

dead as... adj.

[14C+] used in a variety of phrs. meaning absolutely dead (see also combs. below).

In phrases

...a doornail (also dead as a coffin nail, ...hangnail, ...nail) [SE doornail, the large-headed nails with which doors were studded for extra strength and protection; 17C+ use is SE] [mid-14C–16C]