I'm not really surprised that you don't know the entymology of the word, but this might enlighten you.
https://en.wikipedia.org/wiki/Nigger
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Etymology and history
Main article: Negro
The variants neger and negar, derive from the Spanish and Portuguese word negro (black), and from the now-pejorative French nègre (negro). Etymologically, negro, noir, nègre, and nigger ultimately derive from nigrum, the stem of the Latin niger (black) (pronounced [ˈniɡer] which, in every other grammatical case, grammatical gender, and grammatical number besides nominative masculine singular, is nigr-, the r is trilled).
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The root has nothing to do with intelligence. It rises from words describing color and has evolved from various languages.
You seem to be saying that the offended have no right to be offended. That's interesting. I don't happen to agree. Unintentional offenses are the most usual sort. Intentional offense can be avoided if one has the slightest amount of self control.
If it does not hurt you to understand that offense is taken, then why persist in offending? It seems to me that this is simply common courtesy.
By the way, who you are, what you are, where you come from or any other defining feature of your self definition makes no difference in whether or not you have offended someone. If they are offended, they are offended.