Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† hippŏsĕlīnum, i, n., = ἱπποσέλινον, a sort of wild parsley; lit., horse-parsley, Plin. 19, 8, 37, § 124; Pall. Apr. 3 (Col. 11, 3, 36, written as Greek, and translated by olus atrum).