Green’s Dictionary of Slang

1. (US black) guts, courage, integrity, energy.

1992		R. Price Clockers 15: The kid had flex, and flex was rare.
1993		Dr Dre ‘Nuthin But a G Thang’ 🎵 Showin’ much flex when it’s time to wreck a mic.
2002		Simon & Burns ‘The Detail’ Wire ser. 1 ep. 2 [TV script] ‘Why d’you pick him up?’ ‘Harass him,. See what kind of flex he shows’.

2. (W.I./UK black teen) a person’s mannerism, idiosyncrasies etc.

1997

C. Newland Scholar 204: Fuck college [...] I’m fed up o’ havin’ brok pockets all the while, it’s time to change the flex.

3. a variety of street dance.

2014

Observer 13 Apr. 16/2: The world’s greatest exponent of flex, a genre of street dance, Story has named his own form of flex ‘mutant’.