Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
abbās, ātis, m. [avus, avia], the head of an ecclesiastical community, an abbot (eccl. Lat.), Sid. 16, 114; Inscr. Mommsen, 3485 (A. D. 468).
Hence, abbātissa, ae, f., an abbess, Inscr. Mommsen, 3896 (A. D. 570); and abbātĭa, ae, f., an abbey (eccl. Lat.), Hler.