I've had that yelled a few times, usually from teens in an over crowded car.
My favorite was when a car approaching from the rear slowed down. I sensed the driver was trying to equal my speed. This drew my attention to the driver, an attractive woman in a shiny, silver SL550. She was smiling and said, "nice a$$." I don't know if she meant it as a compliment or an insult, nor did I care. It made me laugh.