The extra "M" added-in expresses that extra element of contempt for the other drivers' stupidity, impatience, and/or lack of respect.
A Muthafuckma is anybody who cuts you off and drives slow in the fast lane.
They cause accidents and blame the other driver.
They make sudden turns without using their blinkers, causing you to slam on your brakes.
They drive while texting and hit pedestrians in the crosswalk or the car in front of them.
They drive 15 MPH under the speed limit on curvy two-lane roads; to pass them would be too dangerous.
They get in the wrong lane at the stoplight on purpose because there are less cars in it, so they can cut in ahead of 15 ppl who are in the correct lane when the light turns green.
They speed up when you try to pass them.
2. Muthafuckma is as muthafuckma does. And muthafuckmas are horrible drivers.