Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
octōdĕcim, card. num. adj. [octo-decem], eighteen (mostly post-class, for duodeviginti): tetrachma Attica centum octodecim milia, Liv. 39, 5, 14: cohortes, Front. Strat. 2, 5, 37; Eutr. 1, 1.