When a driver swerves in to the next lane to make an unnecessarily wide turn (or an illegal u-turn), despite the fact that there is plenty of space, needlessly endangering the vehicle to their left or right.
Often paired with other poor driving habits, such as the California roll, a San Diego Stop, or a San Francisco Stop (stopping more than 1/2 car length beyond the white line at a stop light).
Don't you just love it when someone rides the bicycle lane all the way up to the light, then pulls a hard San Diego Swerve while turning right on a red without stopping, nearly side-swiping your stopped car for no fucking reason?