Most of us don’t spend much time thinking about doornails. They're not something you'll find in the modern-day home improvement store, and most folks probably wouldn't even recognize a doornail if one was placed in front of them.
However, when something is extremely, irrevocably dead, we've no problem describing it as "dead as a doornail."
This particular idiom has been in the language for a long time. It goes back to the 1300s, when doornails were actually a thing people used rather than a figurative declaration of doom. They were large, broad-headed nails that were pounded into the heavy wooden doors of the time for strength or decoration or both.
So why were doornails considered especially dead? The answer isn't completely clear. It could just be because they were fairly large inanimate objects that were nailed to doors and unable to move.
There are other, more fun theories though such as the doornail is dead because it was located under the door's knocker and forced to endure centuries of blows to the head. Another theory suggests that doornails were clinched, i.e. bent flat after being driven through wood, thus rendering them unusable.
If doornails aren't your thing, there have been plenty of other English comparisons involving dead things throughout the centuries. You might choose "dead as stone," "dead as a hammer," or "dead as clay." Prefer something animal-related? Then try "dead as herring," "dead as mutton," or "dead as a dodo."
By the 1980s, you could even say "dead as disco," though we know as well as ABBA that disco will never die.
There are other expressions related to the "dead as a" category, but trade "dead" for words like "deaf" and "dumb." To hear more about those, listen to the audio above.
