Lewis & Short

pervĭcax, ācis, adj. [per and root vicof vinco],

  1. I. firm, determined; esp. in a bad sense, stubborn, obstinate, headstrong, wilful (not in Cic. or Cæs.): tu pertinaciam esse, Antiloche, hanc praedicas: Ego pervicaciam aio et eā me uti volo … Nam pervicacem dici me esse et vincere Perfacile patior: pertinacem nil moror? Att. ap. Non. 432, 32 sq. (Trag. Rel. v. 8 Rib.); cf.: adversus peritos pervicax, Tac. H. 1, 26; Ter. Hec. 4, 1, 17: musa, Hor. C. 3, 3, 70: accusatio, Tac. A. 13, 33; 3, 33: pervicacissimi Latinorum, Flor. 1, 11, 11: pervicacioris irae fuit, Curt. 8, 6, 1: homines pervicaci audaciā, pertinaci spe, App. de Deo Socr. p. 43 fin.
    1. B. Of things, steady, unmoved: contra flatus pervicax libra, Plin. 16, 36, 65, § 161.
  2. II. Transf., in a good sense, steadfast, firm: recti pervicax, constans adversus metus, Tac. H. 4, 5.
    Hence, adv.: pervĭ-cācĭter, stoutly, stiffly, stubbornly, obstinately: pervicaciter perstare, Dig. 26, 10, 3.
    Comp.: pervicacius causam belli quaerere, Liv. 42, 14; Tac. A. 4, 42; 13, 54 fin.