The Collaborative International Dictionary of English v.0.48:

Fickle \Fic"kle\, a. [OE. fikel untrustworthy, deceitful, AS.
   ficol, fr. fic, gefic, fraud, deceit; cf. f[=a]cen deceit,
   OS. f?kn, OHG. feichan, Icel. feikn portent. Cf. Fidget.]
   Not fixed or firm; liable to change; unstable; of a
   changeable mind; not firm in opinion or purpose; inconstant;
   capricious; as, Fortune's fickle wheel. --Shak.
   [1913 Webster]

         They know how fickle common lovers are.  --Dryden.

   Syn: Wavering; irresolute; unsettled; vacillating; unstable;
        inconsonant; unsteady; variable; mutable; changeful;
        capricious; veering; shifting.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fickle
    adj 1: marked by erratic changeableness in affections or
           attachments; "fickle friends"; "a flirt's volatile
           affections" [syn: fickle, volatile]
    2: liable to sudden unpredictable change; "erratic behavior";
       "fickle weather"; "mercurial twists of temperament"; "a
       quicksilver character, cool and willful at one moment,
       utterly fragile the next" [syn: erratic, fickle,
       mercurial, quicksilver(a)]

Moby Thesaurus II by Grady Ward, 1.0:

164 Moby Thesaurus words for "fickle":
   adrift, afloat, agnostic, alternating, ambiguous, ambitendent,
   ambivalent, amorphous, at loose ends, brittle, capricious, chancy,
   changeable, changeful, coquettish, corruptible, deciduous,
   derelict, desultory, deviable, dicey, disaffected, disloyal, dizzy,
   double-minded, doubting, dubious, dying, eccentric, ephemeral,
   equivocal, erratic, evanescent, faddish, fading, faithless, false,
   fanciful, fast and loose, fence-sitting, fence-straddling, fitful,
   fleeting, flickering, flighty, flirtatious, flitting, fluctuating,
   fly-by-night, flying, fragile, frail, freakish, frivolous,
   fugacious, fugitive, giddy, hesitant, hesitating, impermanent,
   impetuous, impulsive, incalculable, inconsistent, inconstant,
   indecisive, indemonstrable, infirm, infirm of purpose,
   insubstantial, irregular, irresolute, irresolved, irresponsible,
   light, lubricious, mazy, mercurial, momentary, moody, mortal,
   mugwumpian, mugwumpish, mutable, nondurable, nonpermanent,
   not true to, of bad faith, of two minds, passing, perishable,
   polysemous, quicksilver, rambling, recreant, restless, roving,
   scatterbrained, shapeless, shifting, shifty, short-lived,
   shuffling, skeptical, skittish, spasmodic, spineless,
   temperamental, temporal, temporary, ticklish, touch-and-go, toying,
   transient, transitive, transitory, trothless, unaccountable,
   uncertain, unconfirmable, uncontrolled, unconvinced, undecided,
   undependable, undetermined, undisciplined, undivinable, undurable,
   unenduring, unfaithful, unfixed, unforeseeable, unloyal,
   unpersuaded, unpredictable, unprovable, unreliable, unresolved,
   unrestrained, unsettled, unstable, unstable as water, unstaid,
   unsteadfast, unsteady, unsure, untrue, untrustworthy, unverifiable,
   vacillating, vagrant, variable, versatile, vicissitudinary,
   vicissitudinous, volatile, wandering, wanton, wavering, wavery,
   wavy, wayward, whimsical, wishy-washy