The Collaborative International Dictionary of English v.0.48:

Tumbrel \Tum"brel\, Tumbril \Tum"bril\, n. [OF. tomberel, F.
   tombereau, fr. tomber to fall, to tumble; of Teutonic origin.
   Cf. Tumble.]
   1. A cucking stool for the punishment of scolds.
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   2. A rough cart. --Tusser. --Tatler.
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   3. (Mil.) A cart or carriage with two wheels, which
      accompanies troops or artillery, to convey the tools of
      pioneers, cartridges, and the like.
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   4. A kind of basket or cage of osiers, willows, or the like,
      to hold hay and other food for sheep. [Eng.]
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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):

tumbrel
    n 1: a farm dumpcart for carrying dung; carts of this type were
         used to carry prisoners to the guillotine during the French
         Revolution [syn: tumbrel, tumbril]

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

TUMBREL, punishment. A species of cart; according to Lord Coke, a dung-cart.
     2. This instrument, like the pillory, was used as a means of exposure;
and according to some authorities, it seems to have been synonymous with the
trebucket or ducking stool. 1 Chit. Cr. Law, 797; 3 Inst. 219;  12 Serg. &
Rawle, 220. Vide Com. Dig. h.t.; Burn's Just. Pillory and Tumbrel.