The Collaborative International Dictionary of English v.0.48:

Agnostic \Ag*nos"tic\, a. [Gr. 'a priv. + ? knowing, ? to know.]
   Professing ignorance; involving no dogmatic; pertaining to or
   involving agnosticism. -- Ag*nos"tic*al*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Agnostic \Ag*nos"tic\, n.
   One who professes ignorance, or denies that we have any
   knowledge, save of phenomena; one who supports agnosticism,
   neither affirming nor denying the existence of a personal
   Deity, a future life, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

agnostic
    adj 1: of or pertaining to an agnostic or agnosticism
    2: uncertain of all claims to knowledge [syn: agnostic,
       agnostical] [ant: gnostic]
    n 1: someone who is doubtful or noncommittal about something
         [syn: agnostic, doubter]
    2: a person who claims that they cannot have true knowledge
       about the existence of God (but does not deny that God might
       exist)

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "agnostic":
   Humean, Humist, Pyrrhonic, Pyrrhonist, ambiguous, capricious,
   chancy, changeable, dicey, distrustful, doubter, doubtful,
   doubting, doubting Thomas, dubious, dubitante, equivocal, erratic,
   fickle, from Missouri, hesitant, hesitating, in doubt,
   incalculable, incredulous, indecisive, indemonstrable, irresolute,
   leery, mistrustful, mistrusting, polysemous, questioning, scoffer,
   scrupulous, shy, skeptic, skeptical, suspecting, suspicious,
   touch-and-go, unaccountable, uncertain, unconfirmable, unconvinced,
   undivinable, unforeseeable, unpersuaded, unpredictable, unprovable,
   unsure, untrusting, unverifiable, variable, wary, wavering,
   whimsical