The role self-assigned by/to the person in the passenger or back seat of a car who insists on giving unwanted and endless streams of useless directions, much to the annoyance of the driver.
"Look, I'm driving this car. I know where I'm going and yes, I can see that stop light. I don't need a Naginator."
When the co-pilot in a car is very annoying in the way in which they give directions.
person in passenger seat: Make a u-turn in 500 feet, and then take a left, and don't speed, you're going way too fast! Didn't you see that sign? Damn, you missed the turn, what's wrong with you?!