dead letter
noun
1
- a law or treaty which has not been repealed but is ineffectual or defunct in practice形同虚设的条律（或条约）。
1.1
- figurative a thing which is impractical or obsolete〈喻〉不实际的, 过时的：
theoretical reasoning is a dead letter to a child.
对一个孩子进行理论说理是没有用的。
2
- an unclaimed or undelivered piece of mail死邮件。