a dead letter a law or practice no longer observed.

☞ This phrase was originally used with reference to passages in the biblical epistles in which St Paul compares the life-giving spirit of the New Testament with what he sees as the dead 'letter' of the Mosaic law. Later (until the late 19th century) Dead-letter Office was the name given to the organization that dealt with unclaimed mail or mail that could not be delivered for any reason. The expression has been used metaphorically for an obsolete or unobserved law since the mid 17th century.

2013CNN transcripts Immigration reform is a dead letter because those 80 people in the House are simply not going to pass it.