Source : Webster's Revised Unabridged Dictionary (1913)

Light-minded \Light"-mind`ed\ (-m[imac]nd`[e^]d), a.
   Unsettled; unsteady; volatile; not considerate. --
   {Light"-mind`ed*ness}, n.

Source : WordNet®

light-mindedness
     n : inappropriate levity [syn: {flippancy}]