An interesting look at the way our language usage changes over time can be found in You and Heredity, by Adam Scheinfeld, published by the Frederick A. Stokes Company in New York, 1939.
Chapter XXIV of this volume is titled Sick Minds. It begins with a look at mental illness, as it relates to heredity, and segues from manic-depressives and schizophrenia into feeble-mindedness and subnormal intelligence: "It is the very large class of the higher type of feeble-minded, the morons, which concerns us most. Only by an intelligence test can they be distinguished from persons of normal intelligence."
For the IQ test, persons who scored below 90 were considered below average. For them, there was a scale used to further classify the levels of subnormal intelligence. And this is where we get into archaic usage and how that usage has evolved (devolved?). Apparently, the terms used on this scale were acceptable and without the insulting connotations they carry today. They are, in descending order: Dull, Feeble-minded, Moron, Imbecile, and Idiot. Morons had the distinction of being further defined as either High-grade, Mid-grade, or Low-grade Morons.
We all know how these terms are used today. Insulting as they are in their current-day slang usage, they once constituted valid terminology among mental health professionals. Modern-day equivalents include Borderline Intellectual Functioning, Mild Mental Retardation, Moderate Retardation, Severe Mental Retardation, and Profound Mental Retardation.
Fifty to a hundred years from now, it would be interesting to see if these words hold up in the context of intelligence descriptors. Likely they won’t. Language is, and always has been, constantly changing and evolving in various ways. No reason to believe that process would exclude the IQ scale.