ˌdead ˈletter noun [countable]

1. a law, idea etc that still exists but that people no longer obey

2. a letter that cannot be delivered or returned

[TahlilGaran] Dictionary of Contemporary English ▲

dead letter
an agreement or a law which still exists but which people do not obey or which is not effective any more.
The ceasefire agreement was a dead letter as soon as it was signed since neither side had any intention of keeping to it.

[TahlilGaran] English Idioms Dictionary ▲

dead letter
a piece of mail that cannot be delivered or returned to the sender
The letter with no return address went back to the post office as a dead letter.

[TahlilGaran] English Idioms Dictionary ▲

dead letter
n. phr. An undeliverable letter that ends up in a special office holding such letters.
There is a dead letter office in most major cities.

[TahlilGaran] English Idioms Dictionary ▲