It's been that was at least since I was a kid. The first time I recall hearing it was some time after November, 1963.
I think pretty much all the “conspiracies” have now proven to be true (except for maybe that whole “they’re reptiles” thing). It seems Alex Jones needs to be awarded a Pulitzer for “accuracy in media”.
Indeed, there has long been much evidence of conspiracy filtering out from November 22, 1963 and subsequently. There was much public discussion of details that seemed to prove conspiracy that were exasperatingly beyond the reach of “our” intelligence agencies to minimize their growingly-understood culpability in that event.
On the basis of a now well-known CIA memo in 1967, one of the tactics they began having its assets promulgate the epithet “conspiracy theory” in an organized way. Users of the phrase would seem intelligent, presumably having already captured the larger picture of an event, even while the target of the epithet would almost invariably fall into the paradigm’s position of a jumping-to-conclusions, whacked-out ignoramus.
Since 1990 or so, the paradigm has been ingrained by almost everyone over 10 and has become very effective at changing the course of discussions among both young and old.
Through its, the US populace has been made to drool like Pavlov’s dog just as if their knees were made to jerk from the tap of a doctor’s rubber hammer, even as it dumbed down the reasoning skills of millions of people.
US intel agencies, particularly the CIA have been programming us for many decades. (See also: Operation Mockingbird.)