Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ruscum (rustum), i, n., butcher’sbroom: Ruscus aculeatus, Linn.; Plin. 21, 15, 50, § 86; 27, 100, § 173; 23, 9, 83, § 166; Verg. E. 7, 42; id. G. 2, 413; Col. 10, 374; cf. Fest. p. 262 Müll.