Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† arrĕnĭcum (arrh-, arsĕn-), i, n., = ἀρσενικόν (ἀρρενικόν), arsenic, orpiment. vitia cum chartā et arrhenico sanant, Plin. 28, 15, 60, § 214; 34, 18, 56, § 178 (Vitr. 7, 7, 7 fin. uses for it auripigmentum).