Green’s Dictionary of Slang

1. aware, in the know.

1903		in Cincinnati Enquirer 9 May 13: Joe or Joseph – To get next [HDAS].
1910		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.
1912		D. Runyon ‘A Benighted Inquiry’ 5 Nov. [synd. col.] I’m Joe to the Marquis of Queensbury rules and Reach and Spalding’ s guides.
1913		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.
1915		T.A. Dorgan Indoor Sports 6 Jan. [synd. cartoon] Indoor Sports. Watching a joe salesman in a hick suit [...] peddling the old stuff.
1936		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.

1959

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.