Green’s Dictionary of Slang

1. intimate with [SE post-1800].

1660		J. Tatham Rump II i: And though he was out with my Lord many times, he would be in with you, as the saying is, and please your Highness.
1709		Cibber Rival Fools I i: I was in with all the top Gamesters, and when there was a fat Squire to be fleec’d, I had my Office among them too.
1845		R.S. Surtees Hillingdon Hall I 37: He was in with the players too, and had the entrée of most of the minor theatres about London.
1866		Night Side of N.Y. 80: I am in with a policeman on our beat.
1873		J. O’Connor Wanderings of a Vagabond 278: He’s in with all the detectives, and the heads of police departments, and is influential with many of the police judges.
1887		G. Devol Forty Years a Gambler 198: You go and get in with him, and when you are all ready just give me the old sign.
1891		J.A. Riis How the Other Half Lives 156: The political ‘leader’ of the district, who is ‘in with’ the police.
1906		O.C. Malvery Soul Market 131: ‘Saucy Poll’ [...] was ‘in with’ some of the worst characters among the thieves.
1910		Wodehouse Psmith in the City (1993) 46: He thinks, if he has [a hobby] he might work it to keep in with him.
1922		Joyce Ulysses 85: He’s in with a lowdown crowd, Mr Dedalus snarled.
1930		D. Hammett ‘The First Thin Man’ in Nightmare Town (2001) 376: He’s in solid with the police.
1947		J.F. Powers ‘the eye’ in Prince of Darkness 177: Clyde has got the idee [sic] of taking up a collection for Sleep [...] [I]t is Clyde’s idee and so it is going over pretty big as most of the boys is trying to stay in with Clyde.
1953		K. Tennant Joyful Condemned 47: She’s in with all the higher-ups.
1966		J. Mills Panic in Needle Park (1971) 160: He’s in with a guy we want. You ever hear the name Little Tony? He’s the biggest supplier on the West Side.
1975		Sepe & Telano Cop Team 64: He must be in with the hackie.
1980		J. McClure Spike Island (1981) 413: The people round here. Once you’re in with them, they’re as good as gold.

2. suspicious of a person, getting even with.

1864, 1867, 1870

Hotten Sl. Dict.

3. fashionable, socially aware.

1967–8

Baker et al. CUSS 142: In with it A socially adept person.

4. in comparison with, compared with.

1984

Partridge DSUE (8th edn) 596/1: –1889.