rattle-brained

from The Collaborative International Dictionary of English v.0.48

Rattle-brained \Rat"tle-brained`\ (r[a^]t"t'l*br[=a]nd`), a.
   Giddy; rattle-headed.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

36 Moby Thesaurus words for "rattlebrained":
      blank, brainless, dizzy, empty, empty-headed, empty-minded,
      empty-pated, empty-skulled, featherbrained, featherheaded, flighty,
      fluttery, frivolous, gaga, giddy, giddy-brained, giddy-headed,
      giddy-pated, giddy-witted, harebrain, harebrained, hollow, inane,
      jejune, rattleheaded, rattlepated, scatterbrained, scramblebrained,
      shatterbrained, silly, skittish, thoughtless, vacant, vacuous,
      vapid, witless