Green’s Dictionary of Slang

involved with, esp. a plan or scheme, legal or otherwise.

1886		‘Lady Kate, the Dashing Female Detective’ in Roberts et al. Old Sleuth’s Freaky Female Detectives (1990) 33/2: I was in on the last ‘skylark’.
1887		Lantern (N.O.) 19 Mar. 2: I’m scared to say whether John Fitzpatrick is in on the play or not.
1897		A.H. Lewis Wolfville 54: Tutt an’ Dan Boggs wants in on the play.
1902		E. Townsend Chimmie Fadden and Mr Paul 73: If de police [...] is not in on your gents’ agreement dey would cut rates, and swipe all de business.
1910		‘O. Henry’ ‘Strictly Business’ in Strictly Business (1915) 9: If you want to make this a business partnership, [...] I’m in on it.
1925		D. Hammett ‘The Gutting of Couffignal’ Story Omnibus (1966) 19: And now you got me, and you think I’m in on it.
1930		R. Whitfield Green Ice (1988) 208: I wasn’t in on Carrie Donner’s murder.
1938		D. Runyon ‘All Horse Players Die Broke’ in Runyon on Broadway (1954) 701: Maybe you are not in on anything.
1948		H. McCoy Kiss Tomorrow Goodbye in Four Novels (1983) 84: Stop acting like everybody was in on this. Nobody knows about this but you and Cobbett.
1957		R. Prather Always Leave ’Em Dying 156: They were all in on the kill together.
1967		P. Thomas Down These Mean Streets (1970) 244: You’re the kid who was in on that Village stick-up and got shot up.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 169: I don’t want Desperate or Bungalow an’ the chaps in on that one.
1984		A. Sayle Train to Hell 149: I thought he’d finish her off seeing as she was the only other one who was in on it.
1990		in D. Campbell That Was Business, This Is Personal 19: I want a drink out of it [i.e. a robbery] but I’m not in on it.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 257: ‘Now, not many bods are in on this piece of oil.’.
2000		P. Roth Human Stain 221: And Henry is in on it now, that’s clear.