A term of derision (or endearment) formed by joining the word "dumb" and the word "cunt" into a descriptive noun. The term is NOT gender specific, although it is most commonly directed at males, by other males (esp. in Australia).
So then I asked specifically for "nothing else but a single Big Mac, please", and then this dumbcunt asks me if I wanted "just the burger, or the meal deal". What a douche.