That CIA thing about inventing the term conspiracy theory is wrong (a conspiracy??).
My daughter told me that and it didn’t sound correct. So I did a search. Most internet searches come up with an article from 1863 using the first recorded use of the term with regards to British elites supporting the South during the Civil War.
I would have thought it would have been even earlier - from say Roman times and Nero, etc.
The exact term may not have existed in Ancient Rome or the Middle Ages but the concept was there.
This is a lefty article so you can disregard the propaganda but some of the history is interesting:
https://today.usc.edu/conspiracy-theories-past-present-qanon-usc-experts/