The Collaborative International Dictionary of English v.0.48:

Jack-in-a-box \Jack-in-a-box\
   1. (Bot.) A tropical tree (Hernandia sonora), which bears a
      drupe that rattles when dry in the inflated calyx.
      [1913 Webster]

   2. A child's toy, consisting of a box, out of which, when the
      lid is raised, a figure (usually a clown) springs; also
      called jack-in-the-box.
      [1913 Webster]

   3. (Mech.) An epicyclic train of bevel gears for transmitting
      rotary motion to two parts in such a manner that their
      relative rotation may be variable; applied to driving the
      wheels of tricycles, road locomotives, and to cotton
      machinery, etc.; an equation box; a jack frame; -- called
      also compensating gearing.
      [1913 Webster]

   4. A large wooden screw turning in a nut attached to the
      crosspiece of a rude press.
      [1913 Webster]

WordNet (r) 3.0 (2006):

jack-in-the-box
    n 1: plaything consisting of a toy clown that jumps out of a box
         when the lid is opened