dead letter
noun uk/ˌded ˈlet.ər/ us/ˌded ˈlet̬.ɚ/
(MAIL)
[ C ]死信，无法投递的信件
a letter that cannot be delivered to the address written on it and cannot be returned to the person who sent it
the dead-letter office 死信处理办公室
(LAW/AGREEMENT)
[ C usually singular ]已失效的法律;形同虚设的协议;空文
a law or agreement that is no longer effective
The ceasefire treaty was a dead letter as soon as it was signed, as neither side ever had any intention of keeping to it. 由于双方缺乏遵守的诚意，停火协定一经签署就成了废纸一张。