I'm a carpenter and this is how I believe the term came about. Dead as a door nail is when you hammer a nail through a piece of timber and then flatten the end over on the inside so it can’t be removed again (a technique called clinching), the nail is said to be dead, because you can’t use it again. Doornails would very probably have been subjected to this treatment to give extra strength in the years before screws were available. So they were dead because they’d been clinched. It sounds plausible, but whether it’s right or not we will probably never know.
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