The Collaborative International Dictionary of English v.0.48:

featherbrained \feath"er*brained\, feather-brained
\feath"er-brained\, a.
   Giddy; frivolous; lacking seriousness; given to frivolity;
   feather-headed. [Colloq.]

   Syn: airheaded, dizzy, empty-headed, giddy, lightheaded,
        light-headed, silly.
        [1913 Webster + WordNet 1.5]