Someone who must always react to douchebag behavior with equal or greater douchebag behavior. The term generally only applies to people who are otherwise polite and kind until someone is rude to them first.
(Guy#1) Hey man I can't stand Tommy anymore. He has been such an asshole lately. I think I am going to key his car!
(Guy#2) No don't do that dude. Just because he is a douchebag doesn't mean you have to be a douche-back!