The Origin of the Phrase “As Dead as a Doornail”

"In A Christmas Carol, Charles Dickens wrote about the questionable phrase, “dead as a doornail,” saying:

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.

"As with so many etymologies, we don’t know exactly why door nails were used in the phrase rather than something like coffin nails, but we have a reasonably good educated guess. Door nails were long used to strengthen the door. The person building or installing the door would hammer the nail all the way through the boards. On the other side, he would hammer the end flat, bending it so that the nail would be more secure in a process, called “clenching.” In doing so, the nail was rendered unusable for any other purpose. It would be difficult to remove and even more difficult to use again elsewhere. Thus, the bent nail was commonly called “dead” (not just to do with doors, but elsewhere where the nail was bent over and couldn’t be used again.)"

"As to why it is then a “doornail” instead of other cases where such clenching was done, it’s thought it was probably simply because this was commonly done with nails on doors and the euphony of the phrase caused it to stick, where other similar expressions such as “dead as a stone” simply don’t roll off the tongue as nicely."

Another less touted origin theory (read more)