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.) ...Another less touted origin theory is that because of the doornail’s size, particularly the one securing the knocker, it had to be “hit on the head” with a hammer quite a few times more than your average nail. Because of the number of times it was hit, it would certainly be “dead” by the time the head was flush to the wood of the door—that is, if it had been a living thing rather than an inanimate object.
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