"Keep posted" is an idiomatic English expression that implies that one person will keep another up-to-date with information on a particular situation. The phrase is more commonly used as "to be kept posted" or "keep me posted." The former expression refers to a continuing state of an exchange of news. The latter phrase is usually used as a directive to someone of lower status, such as an employee, to keep the issuer informed of future changes. The expression "I'll keep you posted" is used as a promise to another to share news as it happens.
The term "post," however used, refers to shared information. It could have several origins. For instance, the earliest Old English use denoting a long stick of wood was borrowed from the Latin term "postis" of the same meaning. Gradually a "post" began to refer to both the pole on which the sign was erected and the sign itself. The transitive verb "to post" developed from the means by which a sign or "poster" was displayed.
Hence, the Middle English use of the term meaning to display news or names in a public forum was derived. "Posting banns," publicly denouncing someone as "posted," and "posted" as in trespassing signs came about during this period and again refer to the public sharing of news. The term has even continued in its evolution and has been adopted again since the advent of the Internet, as in "posting a reply," a "post" or even the "repost" of a message. Indeed, the Internet and electronic posts have allowed people to "keep posted" and abreast of news and information at an even faster rate than printed posts.
The term "keep posted" and its relationship to a post office or postal mail might be another source of its meaning. A combination of the Old French "poste" and the Old Italian "posta" meaning "a relay station for horses, with Vulgar Latin "posta" meaning "a station" clearly relates to the transportation of mail or information along a given route. From the time that news was relayed by mail or "post," a transitive verb developed into the act of posting or mailing a letter. This verb gradually morphed from sealing an envelope and giving it to the mailman to mean to "keep posted" or to be informed of the latest news.