Source : Webster's Revised Unabridged Dictionary (1913)

Half-witted \Half"-wit`ted\, a.
   Weak in intellect; silly.

Source : WordNet®

half-witted
     adj 1: lacking mental capacity and devoid of subtlety [syn: {dim-witted},
             {simple}, {simple-minded}]
     2: mentally deficient [syn: {feebleminded}, {slow-witted}]