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]

WordNet (r) 3.0 (2006):

scatterbrain
    n 1: a flighty and disorganized person [syn: scatterbrain,
         forgetful person]

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