Thanks for stepping up, Octavia.
I tried looking it up through The Oxford English Dictionary online but you have to subscribe to access it--I think that's why link 18 didn't work--shouldn't online dictionaries be free? SMH. it's like a conspiracy in itself, making it difficult to look up the actual definition of words so it's easier for the PTB to manipulate the language. That's my theory, anyway ;)
However, I was able confirm the claim that the phrase was first used in 1871 through https://www.merriam-webster.com/dictionary/conspiracy%20theory