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Also, dead as a dodo or herring. Totally or assuredly dead; Also finished. For example, The cop announced that the body in the dumpster was dead as a doornail, or The radicalism she professed in her adolescence is now dead as a dodo, or The Equal Rights Amendment appears to be dead as a herring. The first, oldest, and most common of these similes, all of which can be applied literally to persons or, more often today, to issues, involves doornail, dating from about 1350. Its meaning is disputed but most likely it referred to the costly metal nails hammered into the outer doors of the wealthy (most people used the much cheaper wooden pegs), which were clinched on the inside of the door and therefore were "dead," that is, could not be used again. Dead as a herring dates from the 16th century and no doubt alludes to the bad smell this dead fish gives off, making its death quite obvious. Dead as a dodo, referring to the extinct bird, dates from the early 1900s.
American Heritage Idioms
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Phrase(s): dead as a doornail
Go to *dead as a dodo and *dead as a doornail; deader than a doornail
McGrawhill's American Idioms And Phrasal Verbs