Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ŏpŭlus, i, f., a kind of maple-tree: in arboribus, quas vocant opulos, Varr. R. R. 1, 8; Col. 5, 6, 4; 5, 7, 1 al.; Plin. 14, 1, 3, § 12.