Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sexennis, e, adj. [sex-annus], of six years, six years old: erus, Plaut. Poen. 4, 2, 80; 5, 2, 27: cervi, Plin. 8, 32, 50, § 116: sexenni die, after six years, an interval of six years, Caes. B. C. 3, 20.