I was wondering when the word ‘Senpai’ started being used.

I’ve been slowly conceiving a story in my head for the past year or so, currently it doesn’t have a specific period it’s set in, just that it’s before guns existed. I had a character use senpai to refer to someone, but I realized that I didn’t know when the word was created, so I might have to make some changes to be accurate to whatever time period I chose. I did a bit of research on the word, but I couldn’t find anything.