Dead as a doornail is a popular enough expression, although nobody seems to know where in the heck it came from. According to our old friend the Oxford English Dictionary, the expression dates back at least to 1350. Shakespeare himself used it about 250 years later in Henry VI:
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.
— The Second part of Henry VI, Act IV, Scene 10
It's clear that the expression means utterly, obviously, irreconcilably dead. It's not clear why, though, and it's too late to ask Will for his opinion. A casual Google search brings up dozens of amateur attempts at etymology, none of which I particularly trust. According to most of these theories, the expression comes from the days when metal nails were much more valuable than they are today, since the 14th century pre-dates the Industrial Revolution in general and mass production specifically. Back in þe olden days, you didn't waste nails, no sir, you pulled them back out for re-use when you were finished with them.
Except you couldn't do that with door nails. Door nails were permanent installations due to the increased mobility of a door as compared with, e.g. a wall, you didn't want them working loose over time. So to prevent this, door nails were clinched, that is, driven in and then bent over so they couldn't be removed. Ever. Because Dremel rotary tools hadn't been invented yet. This meant the nails were dead — they couldn't be removed and reused.
Which nails specifically were killed in this fashion is up for debate. I've heard nails used for the hinges, nails used to hold the slats that formed the actual body of the door together, and nails that just held in the door knocker. The illustrious Webster 1913 seems to support a version of the latter theory, defining doornail as specifically the nail which acts as a sounding-board for the knocker. Why this nail would be considered the most dead is beyond my comprehension.
Other sources have even less likely stories, not involving clinching but rather the distinctive sound a bad nail made when it fell on the floor, or was hit repeatedly by the knocker, or other such malarky.
Doubtless the phrase became popular due to its catchy alliteration. And, heck, nobody really worries too much what most expressions mean anyway, which is why people say silly things like "I could care less" and "that begs the related question x". There are dozens of similar expressions, some of which make slightly more sense (dead as a stone, dead as a dodo), and some less (dead as a herring, dead as a doorknob).
But if I don't know where the expression came from, at least I'm in good company. Charles Dickens himself wrote in his famous work A Christmas Carol that "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."