dead letter

dead letter idiom
  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 © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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