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:

Cucking stool \Cuck"ing stool`\ (k?k"?ng st??l`). [Cf. AS.
   scealfingst[=o]l, a word of similar meaning, allied to
   scealfor a diver, mergus avis; or possibly from F. coquine a
   hussy, slut, jade, f. of coquin, OE. cokin, a rascal; or cf.
   Icel. k?ka to dung, k?kr dung, the name being given as to a
   disgracing or infamous punishment.]
   A kind of chair formerly used for punishing scolds, and also
   dishonest tradesmen, by fastening them in it, usually in
   front of their doors, to be pelted and hooted at by the mob,
   but sometimes to be taken to the water and ducked; -- called
   also a castigatory, a tumbrel, and a trebuchet; and
   often, but not so correctly, a ducking stool. --Sir. W.
   Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trebuchet
    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]