Green’s Dictionary of Slang

fashionable; thus in crowd/place/thing, the people/place/object of the moment.

1965		L. Bruce How to Talk Dirty 94: There was already this ‘in’ kind of thing with all these musicians.
1968		K. Brasselle Cannibals 223: I slowed down on Sixth Avenue [...] A lot of ‘in’ shops are there.
1968		‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 23: in crowd – the happening people who know where it’s at because they are where it’s at.
1978		F. Norman Dead Butler Caper 49: It’s a perfectly divine little bistro in Chelsea – the in place.
1980		E. Folb Runnin’ Down Some Lines xvii: I had to have a fight in order to get in with the ‘in’ crowd.
1984		C. White Life and Times of Little Richard 193: It was the in-thing to be. Everybody wants to be gay today.
2001		(con. late 1940s) V. Foot Sixteen Shillings And Tuppence Ha’penny 121: Strutting their stuff was the in-phrase.