(a.) Shallow-brained.

birdbrained birdwitted brainless capricious dizzy empty-headed featherheaded fickle flighty fluffy fluttery frivolous frothy gaga giddy giddy-brained giddy-headed giddy-pated giddy-witted harebrain harebrained impulsive light rattlebrained rattleheaded rattlepated scatterbrained scramblebrained shallow shallow-headed shallow-minded shallow-pated shallow-witted shatterbrained silly skittish superficial thoughtless unprofound volatile whimsical witless