It’s based on usage. R word is tainted now, this might change in the future or not. Language doesn’t care about etymology, only about usage. If the usage is generally negative and implies inferiority of a minority it’s a slur.
Calling someone an idiot doesn’t imply that disabled people are inferior, which makes it ok to use.
axexrx@lemmy.world 1 day ago
See also Moron, imbecile, and cretin.
All Originally medical terms, that then entered comon usage as insults, fell out of favor medically because of that, were rejected by polite society, then after a refractory period and enough replacements, became lower grade insults that were more acceptable for common use.