Meaning of dead letter

dead' let'ter

Pronunciation: [key]
  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 a law, ordinance, etc., that has lost its force but has not been formally repealed or abolished.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 of a general post office.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.