From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Ἄνοια (Strong'S #454 — Noun Feminine — anoia — an'-oy-ah )

lit. signifies "without understanding" (a, negative, nous, "mind"); hence, "folly," or, rather, "senselessness,"  2—Timothy 3:9; in  Luke 6:11 it denotes violent or mad rage, "madness." See Madness. Cp. anoetos, "foolish."

 2—Corinthians 11:1

King James Dictionary [2]

FOL'LY, n. See Fool.

1. Weakness of intellect imbecility of mind. want of understanding.

A fool layeth open his folly.  Proverbs 13 .

2. A weak or absurd act not highly criminal an act which is inconsistent with the dictates of reason, or with the ordinary rules of prudence. In this sense it may be used in the singular, but is generally in the plural. Hence we speak of the follies of youth. 3. An absurd act which is highly sinful any conduct contrary to the laws of God or man sin scandalous crimes that which violates moral precepts and dishonors the offender. Shechem wrought folly in Israel. Achan wrought folly in Israel.  Genesis 34 .  Joshua 7 . 4. Criminal weakness depravity of mind.

Webster's Dictionary [3]

(1): ( n.) The result of a foolish action or enterprise.

(2): ( n.) Scandalous crime; sin; specifically, as applied to a woman, wantonness.

(3): ( n.) A foolish act; an inconsiderate or thoughtless procedure; weak or light-minded conduct; foolery.

(4): ( n.) The state of being foolish; want of good sense; levity, weakness, or derangement of mind.

Charles Buck Theological Dictionary [4]

According to Mr. Locke, consists in the drawing of false conclusions from just principles, by which it is distinguished from madness, which draws just conclusions from false principles. But this seems too confined a definition. Folly, in its most general acceptation, denotes a weakness of intellect or apprehension, or some partial absurdity in sentiment or conduct.

See Evil, Sin