Engaging in fruitless effort or trying to revive an interest which has died out. To insist on talking about something that no one is interested in, or that has already been thoroughly discussed. Similar phrases are ‘beating a dead horse,’ or ‘beating a dead dog.’
According to the Oxford English Dictionary, the first recorded use of the expression in its modern sense was by the English politician John Bright, referring to the Reform Act of 1867, which called for more democratic representation in Parliament. Trying to rouse Parliament from its apathy on the issue, he said in a speech, would be like trying to flog a dead horse to make it pull a load.
I heard this phrase used by my father, who grew up using horses the way we now use motorized vehicles. He was in his fifties before he bought a Model A Ford and built a small garage to house it. No one in our family ever forgot the day that he drove up to the garage and, instead of stepping on the brake, started yelling, “Whoa!” I guess we could have said that he was flogging a dead horse! Or, at least, a non-existent one. The garage door survived.