Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
cerrus, i, f., a kind of oak, Turkey oak, Col. 7, 9, 6; Plin. 16, 5, 6, § 17; 16, 6, 8, § 19; Vitr. 2, 8; Pall. 1, 9, 3; id. Febr. 18, 3.