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]

WordNet (r) 3.0 (2006):

rattlebrained
    adj 1: lacking sense or discretion; "his rattlebrained crackpot
           ideas"; "how rattlepated I am! I've forgotten what I came
           for"- Glenway Westcott [syn: rattlebrained,
           rattlepated, scatterbrained, scatty]

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