"nut with flared sides for turning with the thumb and forefinger;" so called for its shape (see wing (n.) + nut (n.)). Meaning "weird person" recorded by 1989, probably not from the literal sense but from the secondary sense of nut, influenced perhaps by slang senses of wing in wing-ding "wild party," originally "fit, spasm" (1937). An earlier, British, sense of wingnut was "person with large, protruding ears" (1986).
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