From Longman Dictionary of Contemporary Englishdoornaildoor‧nail /ˈdɔːneɪl $ ˈdɔːr-/ noun → (as) dead as a doornail
Examples from the Corpus
doornail• She looked dead as a doornail.• If looks could kill, Dooley Barlowe would have dropped him right there, dead as a doornail.• There we were, messing around with his things, and all the time he was dead as a doornail in Paris. 