Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Rŭbi, ōrum, m., a town in Apulia, now Ruvo, Hor. S. 1, 5, 94.
Hence, Rŭbustī-ni, ōrum, m., the inhabitants of Rubi, Plin. 3, 11, 16, § 105; Front. Colon. p. 127 Goes.