Green’s Dictionary of Slang

1. (orig. US black) sophistication, the prevailing fashion.

1948		Kerouac letter 2 Jan. in Charters I (1995) 141: I heard some of the cats discussing him in the john – cats with beards and artistic-looking manners and real Bohemian hipness, etc.
1958		Kerouac letter 11 Feb. in Charters II (1999) 114: He has great vocal ability to explain beatness and hipness.
1967		in S. Harris Hellhole 126: Cora May admits [...] that her own ‘hipness’ and ‘downness’ didn’t get her far.
1970		B. Oakley Salute to the Great McCarthy 157: An apparition all of a sudden at the oor. ‘Albert! Albert the hip!’.
1999		Guardian Guide 3 July–9 July 89: Massive Attack have managed to remain the epitome of hip.
2000		Indep. Rev. 21 Jan. 14: A calculated avoidance of the mainstream, a deliberate rejection of potentially damaging hipness.

2. a sophisticate, in the context of the beat/drug subculture.

1961		L. Block Diet of Treacle (2008) 49: Squares could turn Hip if they were sick enough, but Hips like you [...] couldn’t return to Squaresville.
1969		C. Sellers Where Have All the Soldiers Gone 114: All the hips weren’t like Cat Yates. There were people who’d give away anything they had.