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Originally, a young boy who carried drinking water to a group of field hands such as a threshing or hay-baling crew. Later, the term also included the man who supplied water to the steam engines powering threshing machines. In that case, the water was transported in a water wagon, which was really just a big tank on wheels. Several hundred gallons of water a day was required per steam engine, and the driver (water monkey) filled the tank from the nearest creek or pond with a hand pump mounted on the wagon.