dead letter

dead-letter的词性：
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 A law, directive, or factor still formally in effect but no longer valid or enforced.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