Green’s Dictionary of Slang

posted adj.1

also posted up
[post v.2 ]

(orig. US) aware, in the know, shrewd.

[US]National Encyc. I. 619: Posted-up is an Americanism for well-informed, thoroughly conversant with [OED].
[US]G.W. Harris ‘Letter from Sut Lovingood’ Nashville Union and American XIX June in Inge (1967) 86: A green cockney, city bred – a ‘posted’ one – who deemed everything he didn’t know as not worth knowing.
[Scot]Chambers’s Journal XIII 22/1: But there is no occasion for us to say, with the Americans, that a man is well ‘posted up’ on a subject, while we can say that he is well informed on it.
J.H. Warren Crying Shame of NY 199: Except to the initiated, the thoroughly-posted in the tricks of these accomplished mendicants, the thing is impossible.
[UK]Daily Tel. 13 Nov. n.p.: Nor may the merest schoolboy be posted up in the dates [F&H].
[US]G. Devol Forty Years a Gambler 38: He was not posted, and did not know why I hit him.
[US]D.U. Sloan Fogy Days, and Now 136: I was posted and asked the old lady how old she was when she married, and this was the sockdolager.
[UK]Sheffield Dly Teleg. 8 Aug. 8/3: Great Northern Stores [...] are not able to keep posted up with present day prices.
[Aus]C. Crowe Aus. Sl. Dict. 60: Posted Up, well informed on the subject.
[US]P.G. McLean ‘A Long Shot’ Variety Stage Eng. Plays 🌐 I’ll mention a few little things you must be posted on.
[US]‘Old Sleuth’ Dock Rats of N.Y. (2006) 64: You have come with us; you are here in our midst, you are posted as to our game, and now what are you going to do about it, Ballard?
[US]Grand Forks Dly Herald (ND) 29 Jan. 6/1: The good things go to people who keep posted [...] study the market, study prices.
R.P.T. Coffin Yankee Coast 321: They are well posted on news and history, on law and industry [DA].
[US]S. Bellow Augie March (1996) 463: She was completely posted on Charlotte, better than a sister.

In phrases

keep someone posted (v.) (also keep someone posted up)

(orig. US) to keep someone up to date or informed.

[US]G.W. Whitman in Civil War Letters 20 May 119: I expect the newspaper correspondents keep you pretty well posted as to our movements.
[US]J. O’Connor Wanderings of a Vagabond 284: He extracted from me a solemn promise to keep him posted up on all my movements in New York.
[UK]Newcastle Courant 9 Feb. 2/5: Ladies keep him posted up in all the changes of fashion.
Ranche & Range (N. Yakima, WA) 31 Aug. 9/3: Every farmer [...] needs this [...] journal to keep him posted on crop conditions.
[UK]Leeds Mercury 7 Jan. 7/2: A man [...] naturaly looks to his daily papers to keep him posted on everything.
[US]A. Berkman Prison Memoirs of an Anarchist (1926) 188: Sent ’em to the scab mills [...] to sling hash for the blacklegs and keep ’im posted on the goings on, see?
[US]Van Loan ‘Modern Judgment of Solomon’ in Old Man Curry 260: Keep me posted on Sergeant Smith.
[US]C. Coe Hooch! 191: Keep me posted on everythin’ that breaks.
[US]W.R. Burnett Iron Man 271: Mandl took a couple of yellow-backs out of his pocket [...] ‘There’s some chicken feed,’ he said; ‘keep me posted.’.
[US]O. Strange Sudden Takes the Trail 167: Keep me posted.
[UK]Wodehouse Jeeves and the Feudal Spirit 38: Authors [...] like to keep you posted about this.
[US]D. Pendleton Executioner (1973) 148: Keep us posted on their activities.
[US]W. Groom Better Times Than These (1979) 34: Of course, you’ll keep me posted.
[US]C. Hiaasen Skin Tight 161: The proverbial doo-doo was getting ready to hit the proverbial fan. I’d appreciate it if you guys would keep me posted.
[Scot]I. Welsh Filth 95: Keep us posted on the movements of yobs like Alex Setterington.
[US]C. Cook Robbers (2001) 78: Keep me posted on the cellular.