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]