Green’s Dictionary of Slang

1. (orig. US) a means of infiltrating otherwise closed groups, usu. those holding power and influence.

1929		D. Hammett ‘Fly Paper’ Story Omnibus (1966) 51: She didn’t mean anything to him but an in to the old man’s pockets.
1933		W.R. Burnett Dark Hazard (1934) 161: Pat’s getting a good play. He’s got a strong in out here.
1939		Goodman & Kolodin Kingdom of Swing 42: aul Ash [...] had heard the band out in California, and thought it was something out of the ordinary. He had an ‘in’ at the hotel and when an opening occurred, he made a deal for us.
1957		C. Himes Imabelle 105: ‘I can take care of you-all. I can cover with the cops. I got ins at the precinct’.
1967		L. Bruce Essential Lenny Bruce 171: The juice / The fix / The schmeer / The In.
1979		H. Feldman et al. Angel Dust 80: It was his ‘in’ with the cemetery gang.
1981		J. Ellroy Brown’s Requiem 209: I needed an ‘in,’ someone who knew Ralston and his modus operandi.
1994		Calt I’d Rather Be the Devil 234: Outsiders were horrified by the feudal control the Delta planter enjoyed over his tenants: James found it reassuring, provided that he had an ‘in’ with the boss.
2000		M. Collins Keepers of Truth 136: I got an in with Ronny’s estranged.
2013		K. Sampson Killing Pool 173: Some chancer from North Wales or somewhere turns up in the hood wanting an in and ends up out for the count.
2021		J. Ellroy Widespread Panic 183: Jimmy Dean’s my ‘in’ on the shoot.

2. (US) a means of infiltrating a building.

1947		C.S. Montanye ‘Frozen Stiff’ in Popular Detective Mar. 🌐 Somebody with an in to the ice cream joint unlocked the outside doors with a company key.
1987		G.W. Clemente Cops are Robbers 153: The others could do nothing; after all, I was their in. [...] With my knowledge of the time lock and access to the building, I could have done the job myself.

3. (US teen) an important, influential, socially admirable individual.

1966		Baltimore Sun (MD) Sun. Mag. 10 July 4/1: The IN is clean shaven, and togged out in a button-down shirt.
1966		Baltimore Sun (MD) Sun. Mag. 10 July 4/3: When an IN asks ‘Where’s the show?’ he means ‘Where is the action tonight?’ .