In modern use, retard is a denigrating term either for someone with a mental disability or for someone who is stupid, slow to understand, or ineffective in some way.[1] The adjective retarded is in the same way used for something very foolish or stupid.[2][3]

It was previously used as a genuine term in medical contexts. The verb "to retard" means to delay or hold back, and so "retard" became known as a medical term in the late 19th and early 20th centuries to describe children with retarded mental development.[4] Up until around the 1960s, the terms "moron", "idiot", "cretin" and "imbecile" were all genuine terms 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.[5][6] Since then, the terms "handicapped" (United States) and "disabled" (United Kingdom) have replaced "retard" and "retarded".