What does deader than a doornail mean?

Answer 1

The phrase means 'Dead beyond doubt'. While the origin of the phrase is still disputed, doornails (circa 1350) are heavily-studded nails used to strengthen exterior passage doors. The expensive homes used them for ornamentation as well. To add strength and prevent theft, the nail was clinched on the inside, making it 'dead', or, unable to be re-used for the same purpose.