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]WordNet (r) 3.0 (2006):
crackbrained adj 1: insanely irresponsible; "an idiotic idea" [syn: crackbrained, idiotic]