Definition: To keep someone informed or up to date on a issue.
Origins: During the Colonial Era, if someone wanted to share news or information with the community, the person would post a note on a large wooden post. The wooden post usually was in a centralized area where people gathered for conversation or gossip. This is where the saying “keep me posted” originated from, because people literally had to nail it on a post.
Charlotte Lowe, History Fact-O-Pedia, New York: Skyhorse Publishing, 2011, 8.