Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
trĕcentēsĭmus, a, um, adj. [contr. from the Gr. ], the three-hundredth: annus, Cic. Rep. 1, 16, 25; Liv. 4, 7, 1; Val. Max. 8, 13 fin.; Eutr. 2, 1.