The Collaborative International Dictionary of English v.0.48:

Skeptic \Skep"tic\, Skeptical \Skep"tic*al\, a. [Written also
   sceptic, sceptical.]
   1. Of or pertaining to a sceptic or skepticism; characterized
      by skepticism; hesitating to admit the certainly of
      doctrines or principles; doubting of everything.
      [1913 Webster]

   2. (Theol.) Doubting or denying the truth of revelation, or
      the sacred Scriptures.
      [1913 Webster]

            The skeptical system subverts the whole foundation
            of morals.                            --R. Hall.
      [1913 Webster] -- Skep"tac*al*ly, adv. --
      Skep"tic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

skeptical
    adj 1: denying or questioning the tenets of especially a
           religion; "a skeptical approach to the nature of
           miracles" [syn: disbelieving, skeptical, sceptical,
           unbelieving]
    2: marked by or given to doubt; "a skeptical attitude"; "a
       skeptical listener" [syn: doubting, questioning,
       skeptical, sceptical]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "skeptical":
   Humean, Pyrrhonic, agnostic, ambiguous, capricious, cautious,
   chancy, changeable, cynical, dicey, disbelieving,
   disinclined to believe, disposed to doubt, dissenting, distrustful,
   doubtful, doubting, dubious, equivocal, erratic, fickle,
   freethinking, from Missouri, guarded, hard of belief, hesitant,
   hesitating, impervious to persuasion, in doubt, incalculable,
   inconvincible, incredulous, indecisive, indemonstrable, irresolute,
   leery, mistrustful, mistrusting, polysemous, questioning,
   scrupulous, shy, shy of belief, skeptic, suspecting, suspicious,
   touch-and-go, unaccountable, unbelieving, uncertain, unconfirmable,
   unconvinced, unconvincible, uncredulous, undivinable,
   unforeseeable, unpersuadable, unpersuaded, unpersuasible,
   unpredictable, unprovable, unsure, untrusting, unverifiable,
   unwilling to accept, variable, wary, wavering, whimsical