keep (someone) posted, to

keep someone posted

Fig. to keep someone informed (of what is happening); to keep someone up to date. If the price of corn goes up, I need to know. Please keep me posted. Keep her posted about the patient's status.
See also: keep, post
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

keep posted

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]
See also: keep, post
The American Heritage® Dictionary of Idioms by Christine Ammer.

keep (someone) posted, to

To supply someone with up-to-date information. This term originated in the United States in the early nineteenth century and comes from accounting, where the latest figures entered into a system are said to be “posted.” It originally was put as being posted up, as in “Mr. M. is not well posted up, or he would have said less on this subject” (The Weekly Oregonian, 1854).
See also: keep
The Dictionary of Clichés by Christine Ammer
See also:
  • keep (someone) in the picture
  • keep (one's) eyes on (someone or something)
  • keep behind
  • keep away
  • keep away (from someone or something)
  • keep faith with
  • keep faith with (someone or something)
  • keep in
  • keep in some place
  • bear/keep somebody/something in mind