The Collaborative International Dictionary of English v.0.48:

Giddy \Gid"dy\, a. [Compar. Giddier; superl. Giddiest.] [OE.
   gidi mad, silly, AS. gidig, of unknown origin, cf. Norw.
   gidda to shake, tremble.]
   [1913 Webster]
   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.
      [1913 Webster]

            By giddy head and staggering legs betrayed. --Tate.
      [1913 Webster]

   2. Promoting or inducing giddiness; as, a giddy height; a
      giddy precipice. --Prior.
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            Upon the giddy footing of the hatches. --Shak.
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   3. Bewildering on account of rapid turning; running round
      with celerity; gyratory; whirling.
      [1913 Webster]

            The giddy motion of the whirling mill. --Pope.
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   4. Characterized by inconstancy; unstable; changeable;
      fickle; wild; thoughtless; heedless. "Giddy, foolish
      hours." --Rowe. "Giddy chance." --Dryden.
      [1913 Webster]

            Young heads are giddy and young hearts are warm.
                                                  --Cowper.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Giddy \Gid"dy\, v. i.
   To reel; to whirl. --Chapman.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Giddy \Gid"dy\, v. t.
   To make dizzy or unsteady. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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, giddy, woozy,
           vertiginous]
    2: lacking seriousness; given to frivolity; "a dizzy blonde";
       "light-headed teenagers"; "silly giggles" [syn: airheaded,
       dizzy, empty-headed, featherbrained, giddy, light-
       headed, lightheaded, silly]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "giddy":
   addled, adrift, afloat, alternating, amorphous, babbling, beery,
   bemused, besotted, blind drunk, brainless, capricious, changeable,
   changeful, crapulent, crapulous, delirious, desultory, deviable,
   dizzy, drenched, drunk, drunken, eccentric, empty-headed, erratic,
   faint, far-gone, fast and loose, featherbrained, featherheaded,
   fickle, fitful, flickering, flighty, flitting, fluctuating,
   flustered, fluttery, fou, freakish, fribble, fribbling, frivolous,
   full, gaga, gay, giddy-brained, giddy-headed, giddy-pated,
   giddy-witted, glorious, happy, harebrain, harebrained, impetuous,
   impulsive, in liquor, incoherent, inconsistent, inconstant,
   indecisive, inebriate, inebriated, inebrious, infirm, intoxicated,
   irregular, irresolute, irresponsible, jolly, light, light-headed,
   lightheaded, maudlin, mazy, mellow, mercurial, merry, moody,
   muddled, nappy, off, rambling, ranting, rattlebrained,
   rattleheaded, rattlepated, raving, reckless, reeling, restless,
   roving, scatterbrained, scramblebrained, shapeless, shatterbrained,
   shifting, shifty, shikker, shuffling, silly, skittish, sodden,
   sotted, spasmodic, spineless, swimming, thoughtless, tiddly, tipsy,
   turned around, unaccountable, uncertain, uncontrolled,
   undependable, under the influence, undisciplined, unfixed,
   unpredictable, unreliable, unrestrained, unsettled, unstable,
   unstable as water, unstaid, unsteadfast, unsteady, vacillating,
   vagrant, variable, vertiginous, vicissitudinary, vicissitudinous,
   volatile, wandering, wanton, wavering, wavery, wavy, wayward,
   whimsical, wishy-washy, witless, woozy, yeasty