Green’s Dictionary of Slang

(orig. US black/teen) unwritten rap lyrics (occas. used of other forms of music), using whatever comes to mind at the spur of the moment.

1983		N.Y. Times 28 Aug. 60/4: The finale — a ‘freestyle’, in which rapping emcees traded improvised rhymes in a verbal jam session [...] brought out members of Grandmaster Flash and the Furious Five .
c.1983		Schoolly D ‘Cuttin and Scratchin’ 🎵 We gonna do a little freestyle for y’all, and it go like this.
1994		Source May 55: If I’m on stage and I forget the words [...] I could just go into a freestyle instead of breaking my flow.
2005		B. Coleman Rakim Told Me 67: ‘[T]hat's just an old freestyle that I used to rock.’ .