A person (more fully live letterbox) or place (more fully dead letterbox) through which spies pass information. Cf. letter drop n. (b) at letter, n.1…

A place, usually secret, where letters, information, etc., may be passed on to, or left to be collected by, another person, as in espionage. slang.

A secret meeting(-place), esp. for the transfer of goods or information.

slang. Esp. in espionage: a person who receives information and either transmits it or holds it for collection; (also) = drop, n. 17d.