The "r slur" is a substitute to saying the word retarded, which is commonly misidentified as a slur. In reality, the reason it's used is that it literally means to delay/ to slow, and not used as an "ableist" term anymore. Eg,
Retarded means slow, literally or figuratively.
Retarded can mean stupid (mentally slow), as in:
He is retarded (slow, not used for disabled people obviously)
What a retard
It can also be used formally as a substitute to the word slow/delay:
The plane's thrust was retarded
These clothes contain a flame retardant
Example of someone who doesn't say the word "retarded":
I don't want to say the word retarded as it's ableist, so I say "r slur"
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