dead letter

plural dead letters

1. An item of mail that cannot be delivered to its intended recipient; after some time it is returned to the sender, or destroyed.
2. A law or other measure that is no longer enforced.

   1905, Basil Hall Chamberlain, Things Japanese, Railways The most formidable obstacle in the way of Japanese railway enterprise is that conflicting interests and local intrigues are apt to render the law of expropriation for public benefit little more than a dead letter.

   3 more examples

   Laws are a dead letter, without courts to expound and define their true meaning and operation.

   1787, Alexander Hamilton, Federalist Papers, Federalist no. 22

   Playing-cards — the devil's picture-books — were hated by the Puritans like the very devil; […] The fine for selling these cards must have been a dead letter, for we find in the newspapers proof of the prevalence of card-playing.

   1894, Alice Morse Earle, Customs and Fashions in Old New England, page 239

   But since, in all these states, the grand juries and the courts refuse to act, the laws are dead letters.

   1932, Harry Haywood, Milton Howard, Lynching: A Weapon of National Oppression, page 12

3. broadlyAnything that has lost its authority or influence despite still being in existence or formally in force.

   I have thought proper to premise thus much, lest the incredible tale I have to tell should be considered rather the raving of a crude imagination, than the positive experience of a mind to which the reveries of fancy have been a dead letter and a nullity.

   1833, Edgar Allan Poe, MS. Found in a Bottle