When the word "darkie" was common, that was the word for someone whose skin was darker than that of the person referring to them. My grandparents would have referred to "darkies" but they didn't use it as condemnation - just as a label to differentiate them.
It was because "darkies" were in some places considered inferior (eg. in places where they were usually slaves doing very menial work) that the word was disliked and replaced by others. But the underlying attitude to them remained, so the new word became tainted with the contempt, so it was itself replaced.
If someone doesn't refer to "?????" very often, then they don't know that the word they are accustomed to has acquired the connotations of the one that it replaced.
The usual answer to that reason for the previous name being used is that "They ought to keep up!". But there are thousands (tens of thousands? millions?) of ethnic or other groups in the world. The acceptable terms can change over a very short time.
I repeat what I said above - if the attitude to "different" people could change, it wouldn't matter what word was used for them, "following the spirit of the law, not the letter."
As the MSM cannot resist calling out Tommy Robinson’s real name, why does Polanski get a free pass?


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