Green’s Dictionary of Slang

(orig. US black/teen) of (rap) music, in a spontaneous manner.

2000		M. Manning Get Your Cock Out 19: The band started jamming freestyle, a kaleidoscopic space metal boogie from the stars.
2005		posting at www.proz.com 🌐 Rapping freestyle is off the top of your head (if you rap) and people who are skilled in it can take part in ‘rap battles’.