There have been for many years at least three different terms commonly used to apply to persons of defective mentality. These were idiot, imbecile and feebleminded. Each in its turn had originally been applied as a very kindly designation of mentally deficient people. Idiot, which sounds so harsh to-day, was originally taken over from the Greek language "idiotes," meaning having an individuality of his own, or in a sense peculiar, not an obnoxious term to be applied to a serious mental defective. But of course in time it came to take its meaning from that to which it was applied. Likewise, imbecile, which means literally leaning upon a staff or needing support, was also a friendly term. Still more recently, the expression feebleminded has come to be applied to these people with the result that it is coming to be a little unpleasant in its implication.Goddard appears to have popularized if not invented "moron", which came to have the same negative connotations as the others (and indeed the OED cites many such for "idiot". Oh, and "idiot" was also defined as "natural fool", which in fact it had replaced in legal terminology.
According to Robert B. Edgerton, in Mental Retardation, "In the past an idiot was someone with an IQ of less than 30, an imbecile had an IQ of 30 to 50, and a moron an IQ of 50 to 70." These terms were replaced by various levels of "retardation", and "retard" is showing signs of becoming taboo. Apparently the preferred label is "intellectually disabled", but perhaps that will be replaced by "exceptional". It may be necessary to change terms
periodically once they have taken on destructive connotations.