turtle

n. to be the main driver or source of transportation for a person or group of people even if you're not invited or are unable to attend whatever event they need the ride for; usually boss, older brother, or person in authority over you so that you are unable to decline
derived from the character turtle from the show entourage

It sucks being the turtle for my older brothers whenever they want to go clubbing. I can't wait till I'm 21.