Green’s Dictionary of Slang

(US) aware, in the know; thus as n., one who is aware.

1896		F. Hutcheson Barkeep Stories 142: ‘I never see him afore widout de harness on, an’ dat’s w’y I ain’t Joseph to him’.
1908		N.Y. Globe 25 July in Fleming Unforgettable Season (1981) 136: The various officials [...] always bar the way to those who are not Joseph to their elaborate system.
1909		F.H. Tillotson How I Became a Detective 86: One thief may say to his pal, ‘Are you hept?’ or ‘Are you Joseph?’ or ‘Are you jo hept?’.
1920		Ade Hand-made Fables 112: He became Joseph to the Cruel Knowledge that his swell Moniker [...] was not bringing in any Jack.