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. Back in Old English times, when the word fat was emerging, food was a lot more scarce than it is today, and the word fat meant simply plump or well-fed. Times have changed, and now that we have more food than we know what to do with, fat people are thought to lack self control, which makes them seem foolish, or even fatuous, which is hardly the case.
