Baka (stupid, idiot) is about as nasty a term as one can say in Japanese about someone else. The phrase would mean something like "it's stupid" or "you're an idiot."
Interestingly enough, the word is written with the kanji for horse and deer. In a sense, it is like calling someone an ass (i.e., donkey, not the derriere, which technically is "arse") in English, though the Japanese don't think of the horse or the deer as being stubbornly stupid, the way English speakers think of the jackass.
So the worst thing you could say about someone in Japanese is that he is a horse-deer?