a dead letter
1. A letter that is unable to be delivered to the recipient or returned to the sender. The post office must destroy a dead letter if it can't be delivered or returned after a certain period of time.
2. An existing law or agreement that is no longer obeyed or enforced. The state's Sunday law is a dead letter as many stores open and conduct business on that day.
a dead letter
If a law or agreement is a dead letter, people do not pay any attention to it, although it still exists. In this conflict, international humanitarian law is a dead letter. Note: A dead letter is a letter that the post office is unable either to deliver or to return to the sender, because it does not have the right addresses.
a dead lettera law or practice no longer observed.
This phrase was originally used with reference to passages in the biblical epistles in which St Paul compares the life-giving spirit of the New Testament with what he sees as the dead ‘letter’ of the Mosaic law. Later (until the late 19th century) Dead-letter Office was the name given to the organization that dealt with unclaimed mail or mail that could not be delivered for any reason. The expression has been used metaphorically for an obsolete or unobserved law since the mid 17th century.
1998 Spectator They were saying on the news…that some provision of the Stormont agreement might end up a dead letter.