Last time I checked, "you're an idiot" is an insult. Maybe not one of the worse ones though, I don't know.
I know how to say loco (crazy) but again, saying "you're crazy" is only a light insult.
They won't teach you swear words in school. That's a problem. Because you need to learn all the words, not simply the convenient ones.
On JapanesePod101.com in one lesson they said "We'll teach you how to say 'I'm feeling BAD' a little later" - argh, annoying, why not teach "I'm good" (genki) and "I'm bad" (warui?) at the same time.
In my Japanese class yesterday the teacher ACTUALLY DID tell us the word for "ugly" (opposite of kirei / pretty). Unbelievable. It's not in the textbook vocab list though. Teach did tell us "the word is too strong" though.
Next: Japanese learners can check out
hiragana.jp - will add the furigana to all kanji on the page. Doesn't work with flash sites like the Knights in the Knightmare site but will work with sting.co.jp. There's a firefox extension called
Rikaichan that helps with reading Japanese websites.