conspiracy(n.)
‘mid-14c., "a plotting of evil, unlawful design; a combination of persons for an evil purpose," from Anglo-French conspiracie, Old French conspiracie "conspiracy, plot," from Latin conspirationem (nominative conspiratio) "agreement, union, unanimity," noun of action from past-participle stem of conspirare "to agree, unite, plot," literally "to breathe together" (see conspire)’.
https://www.etymonline.com/word/conspiracy
So there you have it, nothing is new under the sun. According to this, they were talking about conspiracy theories by 1937, but maybe even earlier.