What is the origin of "Deader than a door nail?"

[The_Chao_Goes_Mu]

My girlfriend an I were trying to figure out where the expression
“Deader than a door nail” originated. What does it mean exactly?
I read once and dammit all, I cannot recall where, that it was to actually be “Deader than a Doorn Ale” I have no idea of the validity of this and even if it is true, again, what does it mean?
My brain is racked.

[Q.E.D]

The phrase is actually “dead as a doornail”. It’s from Shakespeare:

From Shakespeare’s King Henry VI.

CADE: 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.

[yoyodyne]

http://www.word-detective.com/012000.html#doornail

[Northern_Piper]

See also the opening of A Christmas Carol:

Marley was dead: to begin with. There is no doubt whatever about that. The register of his burial was signed by the clergyman, the clerk, the undertaker, and the chief mourner. Scrooge signed it. And Scrooge’s name was good upon 'Change, for anything he chose to put his hand to. Old Marley was as dead as a door-nail.

Mind! I don’t mean to say that I know, of my own knowledge, what there is particularly dead about a door-nail. I might have been inclined, myself, to regard a coffin-nail as the deadest piece of ironmongery in the trade. But the wisdom of our ancestors is in the simile; and my unhallowed hands shall not disturb it, or the Country’s done for. You will therefore permit me to repeat, emphatically, that Marley was as dead as a door-nail.

[Exapno_Mapcase]

A similar historical response from World Wide Words:

Q. Dear Word Detective: Would you happen to know the origin of the phrase “dead as a doornail” to describe the recently deceased? I wasn’t even aware that doornails had a pulse! – Allison, via the internet.

A. Well, you never know about inanimate objects – some of them can be remarkably sensitive. I recently had to stop shouting at my TV during the news because I noticed that both its colors and its sound had become muddy and dull. I think I’ve depressed the poor thing. Maybe if I let it show me Mister Rogers for a couple of weeks it’ll cheer up.

“Dead as a doornail,” meaning utterly, completely dead, first appeared in English way back around 1350 A.D. Shakespeare was quite fond of the phrase and used it in several of his plays, probably for the same reason we still use it today: the alliteration of “dead” and “doornail.” Of the alternatives listed by the Oxford English Dictionary (“dead as a herring” and “dead as mutton”), only “dead as the dodo” packs quite the same poetic punch.

As to why a “doornail,” opinions vary a bit. One theory holds that the “doornail” in question was not a nail as we know nails today, but rather a broad, flat plate mounted on the outside of the door to serve as a striking plate for the door knocker. Such a “nail” would be “dead” because it would be fixed tightly to the wood of the door and thus would not ring when struck as metal normally does, but rather give a dull “thump.” This theory is, in the opinion of most authorities, unlikely to be true.

Probably the best theory about “doornail” notes that until the nineteenth century metal nails were both expensive and rarely used, wooden pegs being the norm. Metal nails were used in the construction of doors, however, usually driven clear through the door and then bent over on the other side, rendering them immovable. Such nails were called “dead” in the jargon of carpentry at the time because they could not loosen and work themselves free. “Dead as a doornail” is thus not just a very old saying, but a very old pun as well.