Definition of the word dead letter

The term "dead letter" refers to a piece of mail that cannot be delivered to its intended recipient due to unknown or missing address information, the addressee's refusal to accept the mail, or the addressee's death. The phrase "dead letter" originated in the early 19th century, when letter carriers in England began using it to describe letters that had been returned as undeliverable. The imagery behind the term comes from the fact that in ancient times, letters that could not be delivered were often placed in tombs or buried as a symbol of the person's supposed death or absence. This practice is still reflected in modern times, as postal services today often hold onto dead letters for a period of time before destroying or disposing of them.