After decades of that being used as an insult, 'retarded' is now considered so offensive that some people want it classified as hate speech. As these terms fell into common use as insults, they were replaced by a kinder and gentler term: 'mentally retarded'.
The words 'idiot', 'moron', and 'imbecile' started as clinical terms, referring to people with IQs below 75, 50, and 25, respectively. Some of these examples result from the euphemism treadmill, whereby terms are repeatedly replaced as the previous word falls into such a state of misuse that it cannot be recovered.
