Green’s Dictionary of Slang

1. (orig. US black) to perform music, esp. rap music, well; to create rap lyrics very well.

1993		Source Dec. 45: All these other females there trying to flow and everyone can’t do it.
2003		G. Tate Midnight Lightning 101: With James it just goes and it flows.
2005		B. Coleman Rakim Told Me 194: ‘You didn’t know how I was going to flow from one song to the next’.
2008		‘Grandmaster Flash’ Adventures 106: I asked the crowd if anybody wanted to be upstanding and rap. Creole approached. [...] ‘Lemme flow with it, Flash,’ he bubbled.

2. (US campus) to speak eloquently.

1997–2000

Da Bomb 🌐 11: Flow: Speak very well.