a law, ordinance, etc., that has lost its force but has not been formally repealed or abolished.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.
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]