When a person, usually an aspiring rapper, is freestyling while on the road. Road rhymin' is most effective when accompanied by custom-made beats via in car CD or MP3 player.
Duder 1: "Oh dude I thought of the best rhyme today on the way home from work."
Duder 2: "You was road rhymin' again? Damn man, you gonna get yourself in an accident."
Duder 1: "Fuck all that son! I gots to rhyme!"