The Collaborative International Dictionary of English v.0.48:

Flighty \Flight"y\, a.
   1. Fleeting; swift; transient.
      [1913 Webster]

            The flighty purpose never is o'ertook,
            Unless the deed go with it.           --Shak.
      [1913 Webster]

   2. Indulging in flights, or wild and unrestrained sallies, of
      imagination, humor, caprice, etc.; given to disordered
      fancies and extravagant conduct; volatile; giddy;
      eccentric; slighty delirious.
      [1913 Webster]

            Proofs of my flighty and paradoxical turn of mind.
                                                  --Coleridge.
      [1913 Webster]

            A harsh disciplinarian and a flighty enthusiast.
                                                  --J. S.
                                                  Harford.
      [1913 Webster]

WordNet (r) 3.0 (2006):

flighty
    adj 1: guided by whim and fancy; "flighty young girls" [syn:
           flighty, flyaway, head-in-the-clouds,
           scatterbrained]
    2: unpredictably excitable (especially of horses) [syn:
       skittish, flighty, spooky, nervous]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "flighty":
   abnormal, adrift, afloat, alternating, amorphous, bereft of reason,
   birdbrained, birdwitted, brainsick, buoyant, capricious,
   changeable, changeful, coquettish, crackbrained, cracked, crazed,
   crazy, daft, deluded, demented, deprived of reason, deranged,
   desultory, deviable, dippy, disoriented, distraught, dizzy, dotty,
   eccentric, effervescent, empty-headed, erratic, faddish, fanciful,
   fast and loose, featherbrained, fickle, fitful, flickering,
   flirtatious, flitting, fluctuating, fluffy, freakish, frivolous,
   frothy, gay, giddy, hallucinated, harebrained, impetuous,
   impulsive, inconsistent, inconstant, indecisive, infirm, insane,
   irrational, irregular, irresolute, irresponsible, light,
   light-headed, lively, loco, lunatic, mad, maddened, manic, mazed,
   mazy, mental, mentally deficient, mercurial, meshuggah, moody,
   moon-struck, non compos, non compos mentis, not all there,
   not right, nutty, odd, of unsound mind, off, psycho, queer,
   quicksilver, rambling, rattlebrained, reasonless, reckless,
   restless, roving, scatterbrained, screwy, senseless, shallow,
   shallow-headed, shallow-minded, shallow-pated, shallow-witted,
   shapeless, shifting, shifty, shuffling, sick, silly, skittish,
   spasmodic, spineless, sprightly, stark-mad, stark-staring mad,
   strange, superficial, tetched, thoughtless, touched, toying,
   unaccountable, unbalanced, uncertain, uncontrolled, undependable,
   undisciplined, unfixed, unhinged, unpredictable, unprofound,
   unreliable, unrestrained, unsane, unsettled, unsound, unstable,
   unstable as water, unstaid, unsteadfast, unsteady, vacillating,
   vagrant, variable, versatile, vicissitudinary, vicissitudinous,
   volatile, wandering, wanton, wavering, wavery, wavy, wayward,
   whimsical, wild, wishy-washy, witless