Green’s Dictionary of Slang

1. (orig. US black) involved, aware, informed; doing well, prospering.

1941		‘Digg Mee’ ‘Observation Post’ in N.Y. Age 26 July 9/7: I must admit that my works haven’t been at its best [...] From now on, my column will be ‘in there’.
1953		L. Durst Jives of Dr. Hepcat (1989) 1: He’s just like the man in the casket, dead in there.
1962		Down Beat 13 Sept. 37: A guy playing a horn has [...] gotta get in there.
1995		L. Stavsky et al. A2Z 53/2: in there – successful or happening.

2. (US black) looking attractive.

1942		D. Burley N.Y. Amsterdam Star-News 7 Feb. 16: I guns the scribe [...] Jack, she was truly in there.
1944		Harry Gibson ‘The Hipster’s Blues, Opus 7 ½’ 🎵 Now I’d say this chick is really in there.
1953		L. Durst Jives of Dr. Hepcat (1989) 7: And little sugar when he blows uptown all the joes in the flick do the chill because he’s naturally in there.

3. sexually successful.

1949–51		in M. Daly Profile of Youth 233: A boy with a smooth date is ‘in there like a Teddy Bear’.
1989		M. Amis London Fields 110: I want something? I go for it. Me? I’m in there. Boof.
1993		B. Moore Lex. of Cadet Lang. 197: Ivan In-There a mythical and eponymous cadet who gives his name to, and represents, the third stage in a ‘real’ cadet’s seduction of a woman, this stage presumably consisting in the process of plying her with liquor.
1997		‘Q’ Deadmeat 41: As soon as mit set eyes pun Danya me wuz een dere like swim wear.
2001		N. Barlay Hooky Gear 185: Fuckin shit man I was well in there.

4. (US campus) pleased and excited.

1989

P. Munro Sl. U. 30: You passed your driver’s test. You’re in there!