It is a single credit that you get before going out with your friends. It's used for one non-alcoholic escape beverage during the night. (for example a SunnyD or Tropicana) To prevent uncivilised actions, you later regret.
You can win SunnyD credits during the night by doing awesome things.
Martijn is so wasted, I should give him my SunnyD Credit or else he's going to hook up with some random 30+ grenade from the bar.
