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English definition of “dead letter”

dead letter

noun uk   us  

dead letter noun (MAIL)

[C] a letter that cannot be delivered to the address written on it and cannot be returned to the person who sent it: the dead-letter officeThe postal system

dead letter noun (LAW/AGREEMENT)

[C usually singular] a law or agreement that is no longer effective: The ceasefire treaty was a dead letter as soon as it was signed, as neither side ever had any intention of keeping to it.Rules and laws
(Definition of dead letter from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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