a law, ordinance, etc., that has lost its force but has not been formally repealed or abolished.
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.
A matter no longer of concern or currency; a bygone issue (1663+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with dead letter
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
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