Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ătĭus, a, um, adj., name of a Roman gens., e. g. M. Atius Balbus, etc., and Ătĭa, ae, f., the daughter of Atius Balbus, and mother of Augustus, Suet. Aug. 4 and 94; cf. Atys.