Supply with up-to-date information, as in Keep me posted about your new job. This usage alludes to the accounting practice of posting the latest figures in a ledger. [Early 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.