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Adjective: scatterbrained  'ska-tu(r),breynd
  1. Lacking sense or discretion
    "his scatterbrained crackpot ideas";
    - rattlebrained, rattlepated, scatty [Brit, informal]
     
  2. Guided by whim and fancy
    "scatterbrained young girls";
    - flighty, flyaway, head-in-the-clouds [informal], dippy [informal]

See also: daft [Brit, informal], ditsy [N. Amer, informal], ditzy [N. Amer, informal], dumb, foolish, frivolous, imbecile, imbecilic, stupid