From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
featherbrained
adj 1: lacking seriousness; given to frivolity; "a dizzy
blonde"; "light-headed teenagers"; "silly giggles" [syn:
airheaded, dizzy, empty-headed, featherbrained,
giddy, light-headed, lightheaded, silly]