The Collaborative International Dictionary of English v.0.48:

Pertinacious \Per`ti*na"cious\, a. [L. pertinax, -acis; per +
   tenax tenacious. See Per-, and Tenacious.]
   1. Holding or adhering to any opinion, purpose, or design,
      with obstinacy; perversely persistent; obstinate; as,
      pertinacious plotters; a pertinacious beggar.
      [1913 Webster]

   2. Resolute; persevering; constant; steady.
      [1913 Webster]

            Diligence is a steady, constant, and pertinacious
            study.                                --South.
      [1913 Webster]

   Syn: Obstinate; stubborn; inflexible; unyielding; resolute;
        determined; firm; constant; steady.
        [1913 Webster] -- Per`ti*na"cious*ly, adv. --
        Per`ti*na"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

pertinacious
    adj 1: stubbornly unyielding; "dogged persistence"; "dour
           determination"; "the most vocal and pertinacious of all
           the critics"; "a mind not gifted to discover truth but
           tenacious to hold it"- T.S.Eliot; "men tenacious of
           opinion" [syn: dogged, dour, persistent,
           pertinacious, tenacious, unyielding]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "pertinacious":
   assiduous, balking, balky, bigoted, bulldogged, bulldoggish,
   bulletheaded, bullheaded, case-hardened, clamant, clamorous,
   constant, continuing, crying, demanding, diligent, dogged,
   dogmatic, draining, enduring, exacting, exigent, exorbitant,
   extortionate, faithful, fanatic, fixed, game, gamy, grasping,
   gritty, gutsy, gutty, hardheaded, headstrong, immutable,
   importunate, inalterable, indefatigable, indomitable, industrious,
   insistent, instant, intolerant, invincible, lasting, loud, loyal,
   mettlesome, mulish, nervy, never-tiring, obstinate, opinionated,
   overzealous, patient, patient as Job, permanent, perseverant,
   persevering, persistent, persisting, perverse, pigheaded, plodding,
   plucky, plugging, preoccupied, pressing, rapt, red-blooded,
   refractory, relentless, resolute, restive, sedulous, self-willed,
   set, single-minded, sleepless, slogging, spirited, spiritful,
   spunky, stable, steadfast, steady, stiff-necked, strong-willed,
   strongheaded, stubborn, sulky, sullen, taxing, tenacious, tireless,
   tough, unabating, unconquerable, uncooperative, undaunted,
   undiscouraged, undrooping, unfailing, unfaltering, unflagging,
   unflinching, unintermitting, uninterrupted, unnodding,
   unregenerate, unrelaxing, unrelenting, unremitting, unshakable,
   unsleeping, unswerving, untiring, unwavering, unwearied,
   unwearying, unwinking, unyielding, urgent, utterly attentive,
   weariless, willful