Source : Webster's Revised Unabridged Dictionary (1913)

Half nelson \Half nelson\ (Wrestling)
   A hold in which one arm is thrust under the corresponding arm
   of the opponent, generally behind, and the hand placed upon
   the back of his neck. In the

   {full nelson} both hands are so placed.

Source : WordNet®

half nelson
     n : a wrestling hold in which the holder puts an arm under the
         opponent's arm and exerts press on the back of the neck