Taikei no Yuurei wrote: there is nothing stopping Erfworlders from using the word, they just don't, as it is akin to a curse word for them.
That's really not how it's ever described. It's more like Erfworlders know what the word means, but they don't know why or why the heck you would use that word instead of "croak."
Years, kill, etc. are simply alien words that they magically know the meaning of.
Charlie using it means one of two things:
1. Charlie is not from Erfworld.
2. Charlie has spent enough time around Stupidworlders to feel that "kill" is sometimes more appropriate than "croak."
Charlie was familiar with Judy, so it's possible he learned it from her.
It's also possible he has some means of looking at Stupidworld, and mimics things he sees there (Charles deGaulle, guns, etc. etc.).
It is likewise possible that he's actually a Stupidworlder, but he doesn't want anyone to know that, to the extent that he even hides his "background" as a carny. If anyone ever dug up his past as a Carny Overlord, the difficulty of finding that history would make them think there was nothing left to dig up, and would therefore accept that there's nothing behind that. That kind of lies-within-lies game seems perfectly within character for Charlie, and is reminiscent of the actual Wizard of Oz's efforts to hide his true nature.
The case for Charlie as a Stupidworlder is fourfold:
1. Charlie has used Stupidworlder words in conversation with Erfworlders.
2. Charlie gets Parson's jokes, ie, he sees the Signamancy of Erfworld for what it is.
3. Charlie was the Wizard in Erfworld's version of the Wizard of Oz, who was in fact from Kansas/Stupidworld in the original story. The signamancy of his role therefore suggests Charlie may also not be from Oz/Erfworld
4. Charlie formulated the "first" Perfect Warlord spell. While this could have simply been a bright idea on his part, it could also mean he was aware of the possibility of pulling people from worlds perfectly suited to such a role.
None of these points are conclusive, but all are suggestive.
The case for Charlie as an Erfworlder is twofold:
1. We know Charlie has a history as a Carny Overlord, whose story made no suggestion he was any more than that.
2. Charlie is really good to carnies.
Regarding 1, we only heard the story from Wanda, who heard it from Olive/The Haffaton Casters/Judy. For none of these characters do we have any indication that any of them were "older" than Charlie, so he could easily have hidden his origin from them.
2 is actually the most interesting evidence to me of Charlie as an Erfworlder. If he were just a Stupidworlder who happened to be a carnymancer (like Parson is a hippiemancer), we wouldn't have much reason to expect him to be so sympathetic to the carnymancers. It makes more sense that he empathizes with their plight as one of them.