Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.