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in·sane / inˈsān/ • adj. in a state of mind that prevents normal perception, behavior, or social interaction; seriously mentally ill: certifying patients as clinically insane he had gone insane. ∎  (of an action or quality) characterized or caused by madness: charging headlong in an insane frenzy his eyes glowing with insane fury. ∎  in a state of extreme annoyance or distraction: a fly whose buzzing had been driving me insane. ∎  (of an action or policy) extremely foolish; irrational or illogical: she had an insane desire to giggle.DERIVATIVES: in·sane·ly adv.