E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One who has all the work but none of the profit; he turns the spit but eats not of the roast. The allusion is to the dog used formerly to turn the spit in roasting. Topsel says, They go into a wheel, which they turn round about with the weight of their bodies, so dilligently . that no drudge . can do the feate more cunningly. (1697.)