From Longman Dictionary of Contemporary Englishdead letterˌdead ˈletter noun [countable] 1 USEFUL#a law, idea etc that still exists but that people no longer obey2 TCMa letter that cannot be delivered or returned
Examples from the Corpus
dead letter• The fact that the postwar treaty had been a dead letter for many years did not worry either party.• An arts education is a dead letter for many students.• Theoretical reasoning is a dead letter to the child unless it is closely anchored to practical issues. 