For context, until the 1960s, the terms moron, idiot, cretin, and imbecile were all genuine, non-offensive terms used, including by psychiatrists, to refer to people with mental intellectual disabilities and low intelligence. These words were discontinued in that form when concerns arose that they had developed negative meanings, with “retard” and “retarded” replacing them.[6][7] After that, the terms “handicapped” (United States) and “disabled” (United Kingdom) replaced “retard” and “retarded”. Disabled is now considered a more polite term than handicapped in the United States as well.
https://en.wikipedia.org/wiki/Retard_(pejorative)