Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† thēsīon or -īum, ii, n., = θησεῖον, a plant, bastard toad-flax: Thesium linophyllum, Linn.; Plin. 21, 17, 67, § 107; 22, 22, 31, § 66.