baka

The only real insult to intelligence in Japanese. This is one of the best examples of how Japanese content themselves of a single words, while Westerners long for extreme diversity. That's a very interesting cultural difference. Moreover, Japanese being so polite and respectful, they couldn't have very offensive words.

In English, "baka" (non or adj.) can mean : stupid, foolish, unintelligent, dumb, dense, brainless, retarded, dull(-witted), slow(-witted), half-witted, simple(-minded), thick, dim, dopey, moro*ic, imbecilic, cretinous, thick, idiotic, silly, absurd, ridiculous, co*k-eyed, daft...
or in nouns : fool, idiot, nitwit, dunce, dullard, dunderhead, dolt, dim-wit, dope, thickhead, nincompoop, etc, etc.