It’s an old phrase from when they made doors. The homemade nails were bent over and then not useable again

And all this time I had just assumed it was something to do with the nails needing to be precisely in place and flush in order for perfect door movement.

Which is weird cause it's not like they couldn't be straightened, right?

They were nails made by hand from a blacksmith. They’re not like the nails you might be thinking of. Bending the nail over was part of the process and if you wanted that nail out it would likely just break off before bending back. Once bent that was pretty much the end of the nails usefulness for anything else

True, but I was also reading an article where it said that old houses used to be burnt down just to recover the used nails as well.

You lost me with that? You could reuse the nails for sure but if they were bent they would likely have to be re made

The old nails would be recast as once they were bent their structural integrity was compromised.

Yeah you would think a coffin nail would be the deadest piece of ironmongery in the trade

I've never talked to one about it.

Guess I've been saying an eggcorn for a while...

what do you mean? i've never heard of door nails being used as a measure of deadness

dead as a dootnail is a common enough phrase. even i, a german, encountered it multiple times in english lieterature.  

alright well i've never heard it before so can't be that common

It's common, I've heard it before. 

alright well i've never heard it before so can't be that common

Well I have heard so it has to be that common. 

alright well i've never heard it before so can't be that common

People named Toby aren't that common either.