The Collaborative International Dictionary of English v.0.48:

Trebuchet \Treb"u*chet\, Trebucket \Tre"buck*et\, n. [OF.
   trebuchet, trebukiet, an engine of war for hurling stones, F.
   tr['e]buchet a gin, trap, a kind of balance, fr. OF.
   trebuchier, trebuquier, to stumble, trip, F. tr['e]bucher.]
   1. A cucking stool; a tumbrel. --Cowell.
      [1913 Webster]

   2. A military engine used in the Middle Ages for throwing
      stones, etc. It acted by means of a great weight fastened
      to the short arm of a lever, which, being let fall, raised
      the end of the long arm with great velocity, hurling
      stones with much force.
      [1913 Webster]
      [1913 Webster]

   3. A kind of balance for weighing. [Obs.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Castigatory \Cas"ti*ga*to*ry\, n.
   An instrument formerly used to punish and correct arrant
   scolds; -- called also a ducking stool, or trebucket.
   --Blacktone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trebucket
    n 1: an engine that provided medieval artillery used during
         sieges; a heavy war engine for hurling large stones and
         other missiles [syn: catapult, arbalest, arbalist,
         ballista, bricole, mangonel, onager, trebuchet,
         trebucket]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

TREBUCKET. The name of an engine of punishment, said to be synonymous with
tumbrel. (q.v.)