dead letter

noun
1.
a law, ordinance, etc., that has lost its force but has not been formally repealed or abolished.
2.
a letter that cannot reach the addressee or be returned to the sender, usually because of incorrect address, and that is sent to and handled in a special division or department (dead-letter office) of a general post office.

Origin:
1570–80

dead-letter, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
dead letter
 
n
1.  a letter that cannot be delivered or returned because it lacks adequate directions
2.  a law or ordinance that is no longer enforced but has not been formally repealed
3.  informal anything considered no longer worthy of consideration

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Idioms & Phrases

dead letter

  1. An unclaimed or undelivered letter that is eventually destroyed or returned to the sender. For example, She moved without leaving a forwarding address, so her mail ended up in the dead letter office. [c. 1700]

  2. A statute or directive that is still valid but in practice is not enforced. For example, The blue laws here are a dead letter; all the stores open on Sundays and holidays. [Second half of 1600s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
If it does not have a return address, it will be returned to the post office dead letter file.
Gradually the pendulum moved to the other extreme, and the common law of trade restraint became largely a dead letter.
Warrant unaccompanied by a duly signed rate bill a dead letter.
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