Green’s Dictionary of Slang

(US black) in rap music, one’s delivery.

1994		Source Oct. 46: People liked my flow but they wasn’t really listenin’ to what I was sayin’.
2004		T.I. ‘Be Easy’ 🎵 My shit bang, spit flames, put niggas to shame / Flow so insane, lyrics sick and deranged.
2007		W.J. Cobb To the Break of Dawn 87: [B]y [...] 1986, the rhyme routines were on their way out. In their absence, the vocal inflection of the rapper (or his flow) could come to the fore. [...]. Flow has two basic characteristics: the division of syllables and the velocity at which they are spoken.