What was Big Willy Shakes going for?
This one's pretty self-explanatory if you ask Shmoop. Dead is dead. There's no coming back from it. It reminds us that you can't use a doornail after it's been hammered and bent into place. You might even call it dead—at least in regards to its purpose.
This is exactly the type of thing Jack Cade would say if you ran into him on the street. He's always looking for a fight. He has no problem killing people throughout Henry VI, Part II for little or no reason at all. So we're not expecting him to go down without a full on melee.
The sad part about this scene is that Cade has no real reason to fight. Iden certainly doesn't provoke him. In fact, Iden says something like "You're stealing from me, man, but let's just let bygones be bygones." And when Cade draws his sword on Iden, Iden still says he doesn't want to fight a weak, withered man.
But it does no good. Cade keeps pushing to fight, and eventually, Iden kills him. The sad part is, it was all completely unnecessary. And at the end of it, Cade is the only one who's as dead as a doornail.
This phrase was pretty popular in Shakespeare's day. People already said it much like we do today. So we can't give Shakespeare all the props for this one, Shmoopers.