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):

full nelson
    n 1: a wrestling hold in which the holder puts both arms under
         the opponent's arms and exerts pressure on the back of the
         neck (illegal in amateur wrestling)