from wiki:
The first time that any form of retard was used to describe mentally disabled people was during the 1960s when "there was a push among disability advocates to use the label mental retardation".[3] This push from advocates was because older terms for the mentally disabled, like moron, imbecile, feeble minded and idiot, had developed negative meanings.[3] Retard was not used to refer to mentally disabled people until 1985. It was widely accepted to refer to people who are mentally disabled as mentally retarded, or as a retard. From there, it turned quickly into a pejorative term, as people began to use it interchangeably with words like stupid, or idiot. Many communities, particularly in North America, regard the word as no longer socially acceptable. The fact that it is still commonly used has led to a continuing debate. A common replacement is the phrase "the r-word".[4