Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
rōbŏrĕus, a, um, adj. [robur], oaken, of oak, oak-: pons, Ov. F. 5, 622: axes, Col. 6, 19, 1; 6, 30, 2: materies, id. 11, 2, 13: ligna, Plin. 28, 8, 29, § 113.