Stupid, foolish, or lacking intelligence; having a brain without moisture or vitality (an insult in older English).
A compound adjective combining 'dry' and 'brained,' suggesting a brain drained of wit or intelligence. In older humoral theory, 'dry' associated with melancholy and dullness.
Medieval people believed temperament came from bodily humors—a 'dry brain' was literally thought to be less clever because dryness corresponded to melancholy and lack of wit, so this insult was backed by actual pseudo-science.
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