To be ‘as dead as a doornail’ is to be utterly dead, devoid of life (when applied to people, plants or animals) or finished with, unusable (when applied to inanimate objects).

This expression is old – it has been in use since at least the 14th century. There’s a reference to it in print in 1350, a translation by William Langland of the French poem Guillaume de Palerne:

“For but ich haue bote of mi bale I am ded as dorenayl.”