The Collaborative International Dictionary of English v.0.48:

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.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

half nelson
    n 1: a wrestling hold in which the holder puts an arm under the
         opponent's arm and exerts pressure on the back of the neck