1usually wing nut : a nut with wings that provide a grip for the thumb and fingerThe wing nut on the handle is tightened to secure the blade in the measured position.—The Home Tool Kit

2slang : a mentally unsound personEver since his mesmerizing performance as a psychotic cop killer in 1979's The Onion Field, Woods has possessed the image of a driven, angry, over-the-edge wing nut.—Susan Schindehette et al.

3slang : one who advocates extreme measures or changes (as in politics)