Foolishness, n.
1. The quality of being foolish.
2. A foolish practice; an absurdity. The preaching of the cross is to them that perish foolishness. 1 Cor. i. 18.
Shades of meaning
Fernald has no entry of its own for foolishness; 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