| We control what|
you think with
|Said and done|
|Jargon, buzzwords, slogans|
Mental retardation (often abbreviated MR in clinical settings that still use the term) refers to various physiological conditions that inhibit normal intellectual development. Use of the term in clinical settings has declined in recent decades and it has been replaced by the term "intellectual disability" in the USA's most recent Diagnostic and Statistical Manual of Mental Disorders (DSM).
Since mental retardation generally results in what we could call stupidity, it is used as a derogatory insult (generally with a "fucking" in front of it for extra kewl swearing points) against people and concepts considered to be extremely, irredeemably dumb.
Using the word as an insult is now considered by many to be politically incorrect and insensitive to people with mental disabilities. Most prominent is the United States social campaign Spread the Word to End the Word, which requests that people refrain from using the word altogether and compares it to racial epithets like nigger. The campaign, while well-meaning, has met with a mixed reaction. Pragmatists argue that simply banning a particular word doesn't automatically remove the stigmatization of and discrimination against the social group attached to it. This can be directly observed even with the term "retard" itself, which is a middle word in what's known as a "euphemism treadmill" that started with "idiot", "imbecile", or "moron" and has since added "mentally challenged" and "special" as terms that turned derogatory.
Regardless of the campaign's effectiveness, the word is still evocative of childish catch-all name-calling (similar to "gay" as an insult) and if nothing else, should be avoided for that reason. As it is also quite frequently utilized against the actually mentally ill by certain awful groups, especially often in targeting 'SJWs', use of this word should additionally be avoided for that reason.