Source : Webster's Revised Unabridged Dictionary (1913)

Flighty \Flight"y\, a.
   1. Fleeting; swift; transient.

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

   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.

            Proofs of my flighty and paradoxical turn of mind.
                                                  --Coleridge.

            A harsh disciplinarian and a flighty enthusiast.
                                                  --J. S.
                                                  Harford.

Source : WordNet®

flighty
     adj 1: irresponsibly frivolous; "flighty young girls" [syn: {flyaway}]
     2: absent-mindedly irresponsible; "he said I was too flighty to
        be a good supervisor" [syn: {head-in-the-clouds}, {scatterbrained}]
     [also: {flightiest}, {flightier}]