The Collaborative International Dictionary of English v.0.48:

Light-headed \Light"-head`ed\ (l[imac]t"h[e^]d`[e^]d), a.
   1. Disordered in the head; dizzy; feeling faint; delirious.
      [WordNet sense 1] --Walpole.

   Syn: faint, swooning, lightheaded.
        [1913 Webster + WordNet 1.5]

   2. Thoughtless; heedless; volatile; unsteady; fickle; loose;
      lacking seriousness; given to frivolity; as, light-headed
      teenagers. [WordNet sense 2] "Light-headed, weak men."
      --Clarendon.

   Syn: airheaded, dizzy, empty-headed, featherbrained, giddy,
        lightheaded, silly.
        [1913 Webster + WordNet 1.5] -- Light"-head`ed*ness,
        n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

lightheadedness
    n 1: a reeling sensation; a feeling that you are about to fall
         [syn: dizziness, giddiness, lightheadedness,
         vertigo]
    2: a frivolous lack of prudence