Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sescentēsĭmus (less correctly sexc-) a, um, num. ord. adj. [id.], the six hundredth: anno sescentesimo. Cic. Rep. 1, 37, 58 Mai N. cr.: anno Urbis sescentesimo quinquagesimo quinto, Plin. 8, 7, 7, § 19.