Source : Webster's Revised Unabridged Dictionary (1913)

Simple-minded \Sim"ple-mind`ed\, a.
   Artless; guileless; simple-hearted; undesigning;
   unsuspecting; devoid of duplicity. --Blackstone. --
   {Sim"ple-mind`ed*ness}, n.

Source : WordNet®

simple-minded
     adj 1: lacking subtlety and insight; "a simple-minded argument"
     2: lacking mental capacity and devoid of subtlety [syn: {dim-witted},
         {half-witted}, {simple}]