Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† costum, i, n. (access. form, like the Greek, costŏs, i, f., Luc. 9, 917), = κόστος, an Oriental aromatic plant: Costus Arabicus, Linn.; Plin. 12, 12, 25, § 41; Ov. M. 10, 308; Hor. C. 3, 1, 44 al.; freq. used in the preparation of costly unguents, Plin. 13, 1, 2, § 15 sq.; 13, 1, 2, § 18; in offerings, Prop. 4 (5), 6, 5; for the preservation of fruits, Col. 12, 20, 5 al.