1. Literally, a motorist who is a moron.
2. A motorist who drives irresponsibly, at the risk of bodily harm or death to self, passengers, and other motorists. A moronotorist passes on blind curves, passes motorists who are signalling for left turns, drives too fast for visibility and road conditions, ignores warning signs, and neglects ordinary precautions.
Moronotorists on Hwy 395 frequently try to pass you when you're turning left. A lot of them don't wear their seat belts, so they get ejected. Then they get run over by other moronotorists who are driving too fast to see them in time to stop.
Moronotorists would run over the ambulance crew too, so CHP has to stop all traffic going both directions, and the ambulance crew needs squeegees and dust pans to pick up the pieces.