To make up as you go along. This can be applied to rappers. It is also used to call someone out who is either lying to you, or making something up that sounds good as they go along--as in that motherf*ckas freestylin!
Bob: Why do the roads crack and get potholes in the winter?
Suzy: Because the snow gets down in the roads and pushes them apart.
Bob: Quit freestylin suzy, im tryin to find out the real reason, not what u believe be causin it!