dead letter; unenforced law
Refers to a law or regulation that is still on the books but no longer enforced or observed.
That law has become a dead letter.
meaningless writing; empty words
Describes a text or statement that has no real effect or substance, often used for formal documents or promises that are ignored.
His promise ended up being just empty words.
Compound of 死 (death) and 文 (writing, sentence). The exact historical derivation is uncertain, but the word likely emerged as a metaphor for a text that has 'died' in terms of its practical effect.