n. a letter that cannot move through the post office because the addressee does not exist or because the address is wrong or illegible. (Standard English.) : Every now and then they open the dead letters to see if they can figure out who they were meant for.
n. an issue that does not matter anymore. : This contract is a dead letter. Forget it!
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw-Hill Education.
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