Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ruscārĭus (rustārĭus), a, um, adj. [ruscum], of or for butcher’s-broom: falculae, for weeding out butcher’s-broom, Cato, R. R. 11, 4; Varr. R. R. 1, 22, 5.