Person or persons whom ride in a vehicle, but are not in control of the vehicle, but make comments constantly pertaining to the driver or the way he/she is driving.
He/She: OMG we are doing 80 MPH on the freeway.
Driver: What's wrong with that???
He/She: The speed limit is only 65.
Driver: But everybody is doing the same speed.
He/She: And you're texting somebody.
Driver: Quit being such a nagagator, I know how to drive!