Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
pistillum, i, n., and pistillus, i, m., a pounder, pestle of a mortar, Plaut. Aul. 1, 2, 17: semen sinapis in mortarium conicito et pistillis conterito, Col. 12, 55; Plin. 34, 18, 50, § 169: pistillum grande est, Auct. ap. Non. 221, 5 (yet pastillum appears to be the more correct reading).