This word was made popular to the masses through a song by chamillionaire. The conotation, in this song, is one of carrying illegal drugs in your vehicle.
Chamillionaire's song says, "My music so loud. I'm slangin'. They hopin' that they gonna catch me ridin dirty." Slangin' is a reference to dealing narcotics or illegal drugs. -ridding dirty-
by James Day May 14, 2006