Lewis & Short

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.

        1. (β) With inf.: amata relinquere pernix, Hor. A. P. 165.
          Hence, adv.: pernīcĭter, nimbly, quickly, swiftly, Plaut. Am. 5, 1, 63: equo desilire, Liv. 26, 4: exilire, Cat. 61, 8: alites alte perniciterque volantes, Plin. 8, 14, 14, § 36.