Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ătăvĭa, ae, f. [atavus], the mother of a great-great-grandfather (abavus) or greatgreat-grandmother (abavia), a fourth grandmother, opp. to adneptis, Dig. 38, 10, 1; 38, 10, 10.