Douchetard is a combination of douchebag and retard.
It works on so many levels because, when you are speaking to someone who is both a douchebag and a retard, you certainly don't want to lose any more time than you have to.
As an added bonus for montrealers, douchetard is close enough to Couchetard that you'll say it without even noticing.
Derek: Omg, did you see Demi Lovato's new hair?
Alice: Derek, you're such a fucking douchetard.