In the super safe X and Y of course. Those of you who know a bit of German will know that it comes from the word Nutzlich in German (for useful).
So why can't I name my Chespin Nutzli?
I think it is censored because of nazi, and yeah you probably know what it is or nutte which means hooker. Sorry for writing it here would be perfect with a spoiler feature.
Yeah that is gen 5, but i'm quite sure gen 6 has the same censored words.
This might be because part of the word Nutz means something else, or its because of the name linked to the Nazis from World War II. Remember EVERYONE in the world play X & Y, so its likely to offend other country players.