Fatuous means lacking intelligence. When your mother outlaws calling your brother stupid, use fatuous instead.
Fatuous derives from the Latin fatuus meaning "foolish." It sounds like it should have something to do with being fat, but it actually has no relation to size. Fatuus itself comes from a root that also gave us "debate" and this might be a good way to remember it. You want to debate someone who's fatuous, because they are unintelligent, silly and even a bit conceited, so they probably won't be very persuasive debaters. Just don't call them fatuous to their face. Even if they don't know what it means, it's just not nice!