Silly, crazy, or mentally unbalanced; acting in a wild or irrational way.
From 'lunatic,' which derives from Latin 'luna' (moon). Medieval people believed the moon affected mental states, so 'lunar' became 'looney.' The spelling changed as it became informal slang.
It's wild that 'looney' comes from the belief that the moon makes people crazy—while that's scientifically false, the moon actually does affect ocean tides through gravity, so ancient people were sensing *something* real about lunar power!
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