Maybe he could originally come from England and coin the phrase? It's a spin on the Glasgow Grin, after all. It would make for a different explanation.
Oooo, the whole "white samurai" thing... problem always is with that, you risk being accused of ripping off Shogun.
I'm aware that it wasn't around then. But if this is fiction, is it against the rules to create a fictitious character that supposedly comes up with a term, hundreds of years before it shows up, in real history? I'm not clear on what would be considered against these unwritten rules. I just thought it was an interesting idea to go with.
I didn't know that Shogun owned the template. To be honest, I'm not even sure I'm familiar with Shogun. I'm sure I've seen it, in a way and maybe that's why it popped into my head, like that.
Btw I'm sorry if either of those comments came off sarcastic or anything negative. That's not my intention. I realized after writing them, that some of the words I used may come off that way. But I want to be respectful and not start anything with people.
I doubt very many people in medieval Japan would have spoken English, either, but I'm going to assume that's the language the story's going to be written in. Unless you're going for 100% historical accuracy with the language, then some idioms and expressions get a pass on creative license. As long as they effectively communicate what they're trying to say and don't ruin the atmosphere, then go for it. You'll probably figure out whether it fits or not during editing, anyway.
the only rule is that there are no rules - do whatever you wish... however unless its very tongue in cheek introducing anachronisms will get you bad reviews
Well, then why not go all out and give them ray guns. A Chelsea scar would not effectively communicate it better than describing it, especially if one doesn't know what a Chelsea scar is.
Because literal ray guns are neither an idiom nor an expression, nor would they maintain a medieval Japanese atmosphere or effectively communicate much. So basically all the things I said in the post you just quoted, plus Disney would probably sue.
Depending on how he writes it, that may work. Although, OP should probably just take these into consideration, rather than using our suggestions.