fatuity

Fatuity, n.

Etymology: [L. fatuitas, fr. fatuus foolish: cf. F. fatuité Cf. Fatuous.]

Weakness or imbecility of mind; stupidity. Those many forms of popular fatuity. I Taylor.

Shades of meaning

Fernald has no entry of its own for fatuity; he lists it under idiocy.

Where he writes about it:

Fatuity is sometimes used as equivalent to idiocy, but more frequently signifies conceited and excessive foolishness or folly. — from idiocy