A wingnut is a specialized, convenient item of hardware.
Like a normal nut, it has threads on the inside to engage the threads on a bolt or stud. However, instead of flat faces that must be engaged with a wrench (or Mr. Goodpliers), a wingnut has (typically) two large "wings" that allow it to easily be tightened or loosened with the fingers.
 Alternative meaning
Being a wingnut requires a particularly paranoid worldview, teetering on the edge of or falling wholesale into tinfoil hat territory, as wingnuttery causes the victim to refuse to accept any source of information that doesn't back up their prejudices (essentially, wingnuts are almost always authoritarian as well as being politically right-wing). Constant gibbering about the "liberal media" as well as a slavish devotion to cognitive dissonance, conspiracy theories, global warming denialism, psychological projection, and crackpot theories of economics (i.e. Austrian school) are also, if not required, at the very least nearly universal symptoms. This is all usually accompanied by a side dish of a severe persecution complex. Not all wingnuts are racists, misogynists, homophobes, or transphobes, though many racists, misogynists, homophobes, and transphobes are also wingnuts.