Intellectual disability

Intellectual disability (ID), formerly called mental retardation (often abbreviated MR in clinical settings that still use the term), refers to various physiological conditions that slow or limit cognitive development.

The R word
Since mental retardation generally results in what we could call stupidity, it is used as a derogatory insult against people and concepts considered to be extremely, irredeemably bad.

Using the word "retarded" as an insult is now viewed by many to be offensive to people with mental disabilities for 2 reasons:
 * It degrades disability.
 * It's often insulting by comparison. For example, some people consider the saying, "anti-vaxxers are retarded" as insulting to people with genuine intellectual disabilities, since they might be falsely attributed to belonging to idiot anti-vaccine groups.

Spread the word to end the word
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 the N word.

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. Of course, simply choosing not to say a certain word isn't exactly difficult, and it respects the feelings and dignity of people with disabilities.

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 disabled or mentally ill people by certain awful groups, especially often in targeting 'SJWs', use of this word should additionally be avoided for that reason.