Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ăcernus, a, um, adj. [1. ăcer], made of maple: equus trabibus contextus acernis, Verg. A. 2, 112; 9, 87: solio, ib. 8, 178: mensa, Hor. S. 2, 8, 10; cf. Mart. 14, 90: mensae, Ov. Met. 12, 254 al.