dead letter
n.（名词）
1）An unclaimed or undelivered letter that after a period of time is destroyed or returned to the sender by the postal service.
死信：无人认领的或未被送出的信,经过一段时间后被销毁或通过邮政服务送还给发信人
2）A law,directive,or factor still formally in effect but no longer valid or enforced.
空文,失效了的法律：仍然正式起作用的法律、指令或原动力,但不再合法或执行