Lewis & Short

fătŭĭtas, ātis, f. [1. fatuus], foolishness, folly, silliness, fatuity (very rare), Cic. Inv. 2, 32, 99; id. Att. 11, 25, 2; Firm. Math. 8, 19; Vulg. Prov. 16, 22; August. Ep. 65.