In 1967 The CIA released a memo that coined the label "conspiracy theory" and "conspiracy theorists" to attack anyone who challenged the official narrative from the Warren Commission. It's interesting to note that the document is labelled "psych", for psychological operations or disinformation. It's also marked "CS copy" at the bottom, meaning "Clandestine Services" Unit.
This document was requested and released to The New York Times in 1976 via The Freedom of Information Act. Below is the CIA dispatch. Read it for yourself.
After the 1960's the word "conspiracy theory", "conspiracy theorist", and "conspiracy" started having a negative connotation and is enough to silence anyone who questions the official narrative. To this day we still view conspiracy theorists as crazy tinfoil hatters.