Related to karma. People like to believe that when you do something kind and generous you will be rewarded. Similarly, if you've been a real douche (asshole) they believe that karma will cum and thrust his big black dick into your mouth. And CUM again. So what hapens when we get CUM all over our new sweaters without having done anything to deserve it? Thats when you get a douche credit. This means you've already been punished for your future wrongs, so basically a douche credit is a get out of jail free card which you show to karma when you do something bad.
Person A: Fuck dude, why did you just beat the shit out of that helpless grandma and break an egg on that jew's head?
Person B: I have douche credits and I'm gonna use them all! Biatch! Asshole!
(Person B gets hit by a car)
Person A: Fucking idiot.