Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

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

Source : WordNet®

fickle
     adj 1: marked by erratic changeableness in affections or
            attachments; "fickle friends"; "a flirt's volatile
            affections" [syn: {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}, {mercurial}, {quicksilver(a)}]