Source : Webster's Revised Unabridged Dictionary (1913)

Giddy \Gid"dy\, a. [Compar. {Giddier}; superl. {Giddiest}.] [OE.
   gidi mad, silly, AS. gidig, of unknown origin, cf. Norw.
   gidda to shake, tremble.]
   1. Having in the head a sensation of whirling or reeling
      about; having lost the power of preserving the balance of
      the body, and therefore wavering and inclined to fall;
      lightheaded; dizzy.

            By giddy head and staggering legs betrayed. --Tate.

   2. Promoting or inducing giddiness; as, a giddy height; a
      giddy precipice. --Prior.

            Upon the giddy footing of the hatches. --Shak.

   3. Bewildering on account of rapid turning; running round
      with celerity; gyratory; whirling.

            The giddy motion of the whirling mill. --Pope.

   4. Characterized by inconstancy; unstable; changeable;
      fickle; wild; thoughtless; heedless. ``Giddy, foolish
      hours.'' --Rowe. ``Giddy chance.'' --Dryden.

            Young heads are giddy and young hearts are warm.
                                                  --Cowper.

Source : WordNet®

giddiest
     See {giddy}

giddy
     adj 1: having or causing a whirling sensation; liable to falling;
            "had a dizzy spell"; "a dizzy pinnacle"; "had a
            headache and felt giddy"; "a giddy precipice";
            "feeling woozy from the blow on his head"; "a
            vertiginous climb up the face of the cliff" [syn: {dizzy},
             {woozy}, {vertiginous}]
     2: lacking seriousness; given to frivolity; "a dizzy blonde";
        "light-headed teenagers"; "silly giggles" [syn: {airheaded},
         {dizzy}, {empty-headed}, {featherbrained}, {light-headed},
         {lightheaded}, {silly}]
     [also: {giddied}, {giddiest}, {giddier}]