dead letter

  1. a law, practice, etc. no longer enforced or operative but not formally done away with
  2. a letter that cannot be delivered or returned, as because of incorrect address and lack of return address

See dead letter in American Heritage Dictionary 4

noun
  1. An unclaimed or undelivered letter that after a period of time is destroyed or returned to the sender by the postal service.
  2. A law, directive, or factor still formally in effect but no longer valid or enforced.

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