Think how you look in the mirror when you are flossing your teeth - you are trying to show your teeth as much as possible to get at the plaque. This is where this term is derived from. Originally "invented" in the New Orleans projects and popularized by the Cash Money Brothers, namely Juvenile. This term is applied as a way of showing off as you are doing something because you are smiling with a wide grin showing all your teeth, as if you were flossing your teeth. In context: Slim be flossin' as he roll through in his new Caddy.
Slim be flossin' as he roll through in his new Caddy.