Idiom of the day :- Dead letter

Usage, examples and definition of the Idiom :-

A letter that the post office is unable to deliver — maybe it’s addressed incorrectly or the person to whom it’s addressed has moved — is a dead letter. So, too, is a law, an order or a directive that has lost its effectiveness and is no longer used. ‘That order from the boss is a dead letter. Nobody obeyed it anyway.’