GNU Collaborative International Dictionary of English

Found one definition

1.       
   crackbrained, Crack-brained (-brāndˌ), a. Having an impaired intellect; whimsical; extremely foolish; crazy; as, “crackbrained notions”.  Pope.
   
   Syn. -- idiotic.
   
   [1913 Webster + WordNet 1.5 +PJC]