Green’s Dictionary of Slang

1. au fait, aware, knowledgeable, fashionable, up-to-date; thus get with it v., to get oneself informed or up-to-date; in spec. use, a member of the carnival or circus worlds, e.g. in cites 1946, 1953, 2012.

1944		D. Burley Orig. Hbk of Harlem Jive 15: ‘Pops, I’m tops,’ replied Joe Hipp. ‘I’m with it, and I won’t quit it.’.
1946		J.E. Dadswell Hey, Sucker 13: I am what show people call ‘with it’ and ‘for it!’ I’ll just take for granted that you are a town mark.
1947		M.H. Boulware Jive and Sl. n.p.: git wit it: enjoy yourself.
1953		F. Brown Madball (2019) 5: ‘How are the pickled punks?’ Irby asked him. He got an answering if slightly puzzled grin. ‘You with it?’ the talker asked [ibid.] 42: ‘Hell, kid, you’re with it. You don’t got to pay. Just walk on in’.
1956		H. Gold Man Who Was Not With It (1965) 18: I wanted to be with it someday like Grack, all there, hard, dark, and sure of himself.
1960		‘Lord Buckley’ Hiparama of the Classics 10: The kind of a Cat that came on so cool and so wild and so groovy and so with it.
1964		P. Johnson in New Statesman 28 Feb. n.p.: The broken stiletto heels, the shoddy stereotyped ‘with-it’ clothes [etc.].
1966		P. Willmott Adolescent Boys of East London (1969) 41: I like good records like Frank Sinatra, not the sort of ‘pop’ music other boys like [...] I’m not with it.
1978		A. Maupin Tales of the City (1984) 82: The cotton crotch is for with-it women on the go.
1982		P. Barker Union Street 178: Sometimes I thinks she’s a lot more with it than she cracks on.
1999		K. Sampson Powder 34: The show. First rate. Bloody marvellous, actually. Very, ah, with it.
2005		E. Weiner Big Boat to Bye-Bye 7: A lively, with-it array of young people.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 For It — Often paired with ‘With It’. Describes someone who doesn’t travel or work in the carnival but is connected in some way [Ibid.] With It — ‘(I’m) with it’ means ‘I work at this carnival (or at some other carnival)’.
2014		C.T. McNeely ‘Assisted Living’ in ThugLit Sept. [ebook] One day Chuck wouldn’t be so with it.
2023		R. Milward Man-Eating Typewriter 190: I’m with-it/ I’m switched-on’.

2. (US gay) as interj., yes.

1965

Guild Dict. Homosexual Terms 49: with it? (question) (accent is equal on both words), a greeting asking, ‘Are you ready to go (out) cruising?’ (q.v.) or any of the activities of the homosexual world of the evening. Used in reply (as a statement), it means ‘Yes.’.

3. (US black) enjoying some form of success.

1971

R.D. Pharr S.R.O. (1998) 465: ‘We hear you was with it last night, Kingfish [...] Tell us about this fabulous sting everybpody is talking about’.

4. conscious, mentally aware.

2009

D. Lehr Fence 139: She found Mike able to talk, but he was groggy and only ‘semi with it’.