Origin
Beating a dead horse can alternatively be said as "flogging a dead horse". The earliest recorded use of this term was by British politician and orator, John Bright, when he was referring to the Reform Bill of 1867. Parliament was indifferent about the bill, and Bright said in a speech that would try and gather interest in the Reform Bill "would be like trying to flog a dead horse to make it pull a load."
The Oxford English Dictionary says in The Globe had the earliest use for "flogging a dead horse", used in 1872.
Examples of The Phrase Being Used
"Why keep beating a dead horse and bringing up these old subjects that have already been debated over and over."
"I wanted to discuss why the Dreamcast was superior the Nintendo 64 with my friends, but they said talking about it again would be akin to beating a dead horse."
Note: For most old phrases, sayings, and idioms, finding the exact person or date in which a phrase was originated is impossible. What's provided is the earliest known date the phrase or saying was being used in.
Usually the phrase is a quote from an old newspaper, book, poem, play, etc. If it is already being used by a newspaper though, then the saying must already be common knowledge among the people of that time. This means the origin of the saying is older.
For instance, if someone or something from 1660 is being quoted using a particular phrase or saying, you can assume that the saying must be older, as it's already popular and being commonly used in 1660. How far back varies from phrase to phrase.
There are some cases though where determining the origin from an exact person is possible, however these are not common. In short, most of the origins for these sayings give you a date so you can get a rough idea on when these phrases were being used.
