John Flower, in today's Guardian Unlimited
, points out that the first user of the term 'conspiracy theory', in the sense of "a belief that some covert but influential agency is responsible for an unexplained event", appears to have been Karl Popper. The Oxford English Dictionary
, which tries hard to cite the earliest usages for its entries, posted the term in 1997, with the citation
I call it the 'conspiracy theory of society' - the view that an explanation of a social phenomenon consists in the discovery of the men or groups who are interested in the occurrence of this phenomenon.
from Karl Popper's work, The Open Society and its Enemies
(the second edition, published in 1952 by Princeton University Press).
I never knew that.