Green’s Dictionary of Slang

joe adj.

[ety. unknown, but note northern dial. to be joe, to the master, presumably a generic use of the name]

1. aware, in the know.

[US] in Cincinnati Enquirer 9 May 13: Joe or Joseph – To get next [HDAS].
[Aus]Sun. Times (Perth) 24 July 2nd sect. 10/5: [Johnson] met a lot of ‘Joe’ fighters, and showed up like the sun over an oil lamp, but with Jeff you will see him pitted against the best.
[US]D. Runyon ‘A Benighted Inquiry’ 5 Nov. [synd. col.] I’m Joe to the Marquis of Queensbury rules and Reach and Spalding’ s guides.
[US]Day Book (Chicago) 23 Aug. 11/2: I’ll put you jo [sic] whether you can put the finger on him [...] or get tossed outa here on your sconce.
[US]T.A. Dorgan Indoor Sports 6 Jan. [synd. cartoon] Indoor Sports. Watching a joe salesman in a hick suit [...] peddling the old stuff.
Eve. News (Wilkes-Barre, PA) 30 Sept. 4/1: The other cities [...] say the Giants are a Joe team and that the Yanks aren’t such-a-much.

2. used for anything exceptionally strong, large or extraordinary.

[US]E. De Roo Young Wolves 130: I wantcha ta know I appreciate your tip, and I’ll even it up with ya. Ya been a good chum. A real Joe-pal with me.