1.USEFUL#a law, idea etc that still exists but that people no longer obey 形同虚设的法规,空文;名存实亡的观念
2.TCMa letter that cannot be delivered or returned 无法投递[退回]的信,死信
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 artseducation is a dead letter for many students.• Theoreticalreasoning is a dead letter to the child unless it is closely anchored to practical issues.