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1. offensive slang A derogatory term for someone who is insane or psychotic. Some nutjob 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 nutjob.
2. slang An eccentric, wild, or unorthodox person. You're such a nutjob, Tim—I can't believe you ate the entire thing by yourself! My whole family thinks I'm a bit of a nutjob because I practice holistic medicine.