Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
octingenti, ae, a (gen. plur. octingentūm, Liv. 30, 21, 4), card. num. adj. [octocentum], eight hundred, Cic. Planc 25, 60: stadia, id. Ac. 2, 25, 81.