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1. offensive slang A derogatory term for someone who is insane or psychotic. Some nutcase went rambling through the mall this morning wearing nothing but a plastic bag around his waist, screaming incomprehensibly at anyone who went near him. When you reach a certain level of celebrity, you inevitably start getting stalked by the occasional nutcase.
2. slang An eccentric, wild, or unorthodox person. You're such a nutcase, Tim—I can't believe you ate the entire thing by yourself! My whole family thinks I'm a bit of a nutcase because I practice holistic medicine. My friend Jake is a total nutcase, always pulling the most reckless, crazy stunts.
Fig. a crazy person; an irrational person. Bob is acting stranger and stranger. He is turning into a real nut case.