The Collaborative International Dictionary of English v.0.48:

crackbrained \crackbrained\, Crack-brained
\Crack"-brained`\(-br[=a]nd`), a.
   Having an impaired intellect; whimsical; extremely foolish;
   crazy; as, crackbrained notions. --Pope.

   Syn: idiotic.
        [1913 Webster + WordNet 1.5 +PJC]