That's Shakespeare, boys. That's Shakespeare.

I don't mean to say that I know, of my own knowledge, what there is particularly dead about a door-nail.

It's interesting if you look at the etymology/history. Door nails, for whatever reason, were called dead if after you nailed it through the door you then bent the tip down. It's possible they were called that because until you bent it down you could still pull it out and reuse the nail, so it was alive. Once it was bent, because prior to machines screws were a phenomenal pain to make, you couldn't use it again, so it would be dead.