beat a dead horse

The origin of 'beat a dead horse' comes from the mid-19th century, when the practice of beating horses to make them go faster was often viewed as acceptable. To beat a dead horse would be pointless, as it wouldn't be able to go anywhere. An early written form of this popular idiom appears in an 1859 issue of the London paper Watchman and Wesleyan Advertiser: "It was notorious that Mr. Bright was dissatisfied with his winter reform campaign, and rumour said that he had given up his effort with the exclamation that it was like flogging a dead horse."