Folly, n.; pl. Follies.
Etymology: [OE. folie, foli, F. folie, fr. fol, fou, foolish, mad. See Fool.]
1. The state of being foolish; want of good sense; levity, weakness, or derangement of mind.
2. A foolish act; an inconsiderate or thoughtless procedure; weak or light-minded conduct; foolery. What folly 'tis to hazard life for ill. Shak.
3. Scandalous crime; sin; specifically, as applied to a woman, wantonness. [Achan] wrought folly in Israel. Josh. vii. 15. When lovely woman stoops to folly. Goldsmith.
4. The result of a foolish action or enterprise. It is called this man's or that man's "folly," and name of the foolish builder is thus kept alive for long after years. Trench.
Shades of meaning
Fernald has no entry of its own for folly; he lists it under idiocy.
Where he writes about it:
Folly and foolishness denote a want of mental and often of moral balance. — from idiocy
Fatuity is sometimes used as equivalent to idiocy, but more frequently signifies conceited and excessive foolishness or folly. — from idiocy