In 1967, the CIA Created the Label "Conspiracy Theorists" ... to Attack Anyone Who Challenges the "Official" Narrative

Specifically, in April 1967, the CIA wrote a dispatch which coined the term “conspiracy theories” …
and recommended methods for discrediting such theories.
The dispatch was marked “psych” – short for “psychological operations” or disinformation –
and “CS” for the CIA’s “Clandestine Services” unit.

Summarizing the tactics which the CIA dispatch recommended:

Claim that it would be impossible for so many people would keep quiet about such a big conspiracy

Have people friendly to the CIA attack the claims, and point back to “official” reports

Claim that eyewitness testimony is unreliable

Claim that this is all old news, as “no significant new evidence has emerged”

Ignore conspiracy claims unless discussion about them is already too active

Claim that it’s irresponsible to speculate

Accuse theorists of being wedded to and infatuated with their theories

Accuse theorists of being politically motivated

Accuse theorists of having financial interests in promoting conspiracy theories

In other words, the CIA’s clandestine services unit created the arguments for attacking conspiracy theories as unreliable in the 1960s as part of its psychological warfare operations