What does "dead as a doornail" mean — and why is it funny?

informal

Meaning

Completely dead, or — of an object — utterly broken and lifeless.

Where it comes from

Very old, used since the 1300s and made famous by Dickens in A Christmas Carol. A doornail was a heavy stud hammered into a door, perhaps 'dead' because it was clinched flat and could not be reused.

Why it is funny

The humor is that a doornail was never alive to begin with, so calling it dead is faintly absurd. Dickens himself paused in the story to admit he had no idea why a doornail should be the deadest thing around.

Used in a sentence

"I tried to start the car this morning — battery's dead as a doornail."