Sentence:  <WSD>COOP</WSD>, ... A cart, the box of which moves upon its shafts by hinges, by which means it may be emptied of its load without unyoking the horse, S. "The body of the "cowp-cart" is attached to the shafts by a peculiar kind of hinges, which allow of elevating it before, either partially or entirely, to facilitate the discharge of its load backwards, either by degrees into small heaps, or at once, without the trouble of unyoking the shaft horse." Agr. Surv. of Berw. p. 167. As used in the latter sense, the term is obviously from the "v. to Coup", to overturn. 
Meaning of the word marked with <WSD> tags:  A cart which opens at the back to release its load; a tumbril.