Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† postŏmis, ĭdis, f. [perh. from πρόστομος], a barnacle, twitch, an instrument fixed upon the nostrils of unruly horses; hence, transf., a drinking-cup, from its being always at a tippler’s nose (anteclass.), Lucil. ap. Non. 22, 27 (al. prostomides): postomis, ἐπιστομίς, Gloss. Lat. Gr.