Webster's Revised Unabridged Dictionary · 1913

Folly

Folly , noun

[Old English folie, foli, French folie, from 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. — Shakespeare
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