Dead letter (drop)
A dead letter is originally a statute or law that is no longer observed, dates from the mid-1600s. Later, from c. 1700 it referred to unclaimed mail that was kept at the dead letter office or post office. A dead letter drop, sometimes called a dead drop, is a way of leaving secret messages that get collected later upon receipt of secret signals without the communicating parties meeting. It is a method of communication commonly used in espionage and dates in this sense from the early 20th century.