Synonyms for Scatterbrained (same or very similar meaning)
WordNet sense 1 (lacking sense or discretion):
rattlebrained, rattlepated, scattyWordNet sense 2 (devoid of good sense or judgment):
foolishWordNet sense 3 (guided by whim and fancy):
flighty, flyaway, head-in-the-cloudsWordNet sense 4 (not serious in content or attitude or behavior):
frivolousFrom the ODE community, based on WordNetadd/edit