Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
pernix, īcis (abl. pernici, Lucr. 5, 559: pernice, id. 2, 635), adj. [per-nitor, qs. striving or pressing through], nimble, brisk, active, agile, quick, swift, fleet (cf.: levis, praepes, celer, promptus): pernix sum manibus, Plaut. Mil. 3, 1, 36: corporum pernicium, Liv. 28, 20; Verg. A. 11, 718: viginti milibus juvenum velocitate pernicibus, Vell. 2, 34, 1: conjugis adventu pernix Saturnus, Verg. G. 3, 93: pernicis uxor Appuli, Hor. Epod. 2, 41: genus, Tac. H. 2, 13: nuntii, id. ib. 3, 40: pedibus celerem et pernicibus alis, Verg. A. 4, 180: temporis pernicissimi celeritas, Sen. Ep. 108, 27: saltus, Plin. 9, 47, 71, § 154.