The Collaborative International Dictionary of English v.0.48:
Scatter-brain \Scat"ter-brain`\, n.
A giddy or thoughtless person; one incapable of concentration
or attention. [Written also scatter-brains.]
[1913 Webster]
Moby Thesaurus II by Grady Ward, 1.0:
18 Moby Thesaurus words for "scatterbrain":
birdbrain, featherbrain, featherhead, flibbertigibbet, fool,
giddybrain, giddyhead, goose, harebrain, rattlebrain, rattlehead,
rattlepate, scatterbrains, shallowbrain, shallowpate, shatterbrain,
silly, simpleton