Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
com-pernis, e, adj. [perna], with the knees bent inwards, knock-kneed, Lucil. ap. Non. p. 26, 3; Plaut. Fragm. ap. Fest. p. 375, 20 Müll.; Varr. L. L. 9, § 10 Müll.