The Collaborative International Dictionary of English v.0.48:

Jumping jack \Jump"ing jack`\
   1. A toy figure of a man, jointed and made to jump or dance
      by means of strings or sticks attached to it.
      [1913 Webster]

   2. A calisthenic exercise in which a person starts in a
      standing position with both feet together and with the
      arms at the sides, then leaps into the air while moving
      the arms outward and upward so that the hands touch above
      the head, and then returns to the original position by
      reversing the motion of legs and hands.
      [PJC]

WordNet (r) 3.0 (2006):

jumping jack
    n 1: plaything consisting of a toy figure with movable joints
         that can be made to dance by pulling strings