Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ŏbēsĭtas, ātis, f. [obesus], fatness, stoutness, corpulence, obesity (post-Aug.): et obesitas ventris, Suet. Dom. 18; id. Claud. 41; Col. 6, 24.
Of trees: (arbores) laborant obesitate, Plin. 17, 24, 37, § 219.