Totally and assuredly dead; completely finished or defunct. Also 'dead as a dodo' or 'dead as a herring.'
Examples:
- The cop announced that the body in the dumpster was dead as a doornail.
- The radicalism she professed in her adolescence is now dead as a dodo.
Origin: Dating from about 1350; doornail referred to costly nails hammered into outer doors and clinched on the inside, making them 'dead' (unusable again).
Synonyms: dead as a dodo, dead as a herring