I recently ran into an article about the origin of the term “conspiracy theory“.  A recent book has stated that “The term was… put into wide circulation by the CIA to smear and defame people questioning the JFK assassination”.
I didn’t find “conspiracy theory” on Word Detective, Phrase Finder, or World Wide Words. But the article’s author suggests a source I have not used: “you can actually push the date back even farther using a more recently developed tool, Google Books.” He presents a usage in 1870 from a debate over mistreatment of inmates at insane asylums. (There is another striking phrase within the quoted material: “sprinkle hells with rose-water.”)