http://iq-test.learninginfo.org/iq01.htm - 1904, the IQ test is created
"Reader, suppose you were an idiot. And suppose you were a member of Congress. But I repeat myself." [Mark Twain, c.1882]
c.1300, "person so mentally deficient as to be incapable of ordinary reasoning,"
Looks like Idiot was a derogatory term before psychologists got hold of it.
I think you're thinking of "moron," which was apparently invented as a new word for dumb by the IQ scale people in the 1910s.
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