Green’s Dictionary of Slang

inside n.2

also insides

1. (US) information, esp. when privileged.

[US]Ade Artie (1963) 55: Say, you must think I’m a prize gilly to set around here and give up my insides to you about her.
W.H. Smith Promoters v 101: I’ll give it to you straight, for I happen to know the inside [DA].
[US]C. Coe Hooch! 237: How the hell you got the inside on all this stuff, I don’t know.
[US]W. Winchell On Broadway 13 Dec. [synd. col.] The ‘inside’ on the Fred Perry – Helen Vinson explosion and sudden divorce is a New York Socialulu.
[US]R. Prather Scrambled Yeggs 34: Kelly’s stringing along with us on this; nothing breaks till we say go. Turnabout, he gets some of the inside from us.
[US]Ragen & Finston World’s Toughest Prison 804: inside – Information or knowledge shared by only a few.
[US]C. Bukowski Erections, Ejaculations etc. 51: I have more talent than anybody but you’ve got to have the inside!

2. see inside man under inside adj.

In phrases

dark as the inside of a cow (adj.)

see under dark adj.

on the inside

1. in possession of privileged information.

[US]H. Green Mr. Jackson 70: He’s on the inside in a Western mining syndicate.
[US]W. Irwin Confessions of a Con Man 172: I knew better than any one else, because I was on the inside.
[US]Eve. Public Ledger (Phila., PA) 6 Sept. 2/1: ‘Piggy’ Wagner [...] was tipped off by some one ‘on the inside’.
[US]Dos Passos Manhattan Transfer 15: It’s up to us to be on the inside, in the forefront of progress.

2. see inside adj. (1)