It's not... a slur, is it?
“You’re a mean one, Mr. Grinch” The clue is in the song.
I mean, “You’re a mean one, Mr. Assface” still works despite being more of an insult than a proper name
Mr. Assface had plenty of time to change his name legally
Codsworth recognizes Assface as a proper name, but that was in 2077 so maybe we just haven’t gotten there yet, culturally.
The Grinch Song Uncensored is BRUTAL (ft Joel Haver and Trent Lenkarski)

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