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deader than a doornail

Definitively, undeniably dead. (Doornails were hammered in a such a way that they could not be reused.) I poked that squirrel with a stick and, yeah, it's deader than a doornail.
See also: deader, doornail
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

*dead as a dodo

 and *dead as a doornail; deader than a doornail
dead; no longer in existence. (*Also: as ~.) That silly old idea is dead as a dodo. When I tried to start my car this morning, I discovered that the battery was deader than a doornail.
See also: dead, dodo
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

dead as a dodo

If something is as dead as a dodo, it is no longer active or popular. The foreign exchange market was as dead as a dodo. Note: The dodo was a large flightless bird that lived on the islands of Mauritius and Réunion. It became extinct in the late 17th century as a result of hunting and the destruction of its nests by pigs belonging to settlers on the islands.
See also: dead, dodo
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

dead as a (or the) dodo

1 no longer alive. 2 no longer effective, valid, or interesting. informal
The name dodo comes from Portuguese duodo meaning ‘simpleton’. It was applied to the large flightless bird of Mauritius because the bird had no fear of man and so was easily killed, being quickly wiped out by visiting European sailors. The dodo's fate has made it proverbial for something that is long dead and the name has been used metaphorically for an old-fashioned, stupid, or unenlightened person since the 19th century.
2000 John Caughie Television Drama The once pleasant family hour is now as dead as a dodo.
See also: dead, dodo
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
"Color overall in glassware is deader than a door nail," said one tabletop buyer.
That's the way it was in the old days when the dogs and whips were masters, and that's the way it is today when we got stuff in the water and the air we can't even see that can kill us deader than we ever thought we could die.
Dieffenbachia trees, notably one big-eared Amoena, read over my shoulder, Outside, all id deader than a frosted doorknob, but here is our winter greenhouse, the stove fire roars and crackles, happy for the extra oxygen.
Claims based on oral promises are "deader than a doornail," he says.
Time was when many film buffs dismissed local action movies as being 'deader than a dodo bird,' permanently done in by overaged action superstars and their smug producers' failure to keep viewers excited and dynamically entertained.
Audi A4 Audi A4 > Cheap and |Cheerful Although it's a company that's now deader than a dead thing, Saab got their 9-3 very right towards the end of its life.
Not surprisingly there are blown-out blow-dries and cancelled cut and colour appointments all over the place and it's not long before Blades is deader than Ian's dust-gathering restaurant - remember that place?
This astonished Bo, because it contradicted his notion of shotshell ballistics, common among Americans: More shot equals "deader" birds.
The age of ideas is deader than a badger caught in a culler's sight.
leader of Liberty Party Hrant Bagratyan - 2.15% or 31,643, the deader of National Self- Determination Union Paruyr Hayrikyan - 1.23%
soap is dead.It's deader than the look in Jeremy Kyle's eyes.
Back home in Pennsylvania, February and March make up the proverbial "dead of winter," and this year has been deader than most; unusually cold and snowy.
The .375/06 JDJ with a 270 grain at 2,400 fps won't kill a deer any deader but will be entirely adequate for Elk at that distance--at the expense of moderately heavy recoil and heavy muzzle blast.
ITV's defibrillator Simon Cowell (without him the channel would be even deader on it's a**e than it already is) must have more tricks up his sleeves to keep things moving.