Is the Korean word “in-ma (인마)” a bad word?

While I was talking to my nephew and ended a sentence with the Korean word “in-ma (인마)”, he told me not to use that word because it’s bad. Is it? Since when? Well, it can be a little bit, but it’s more like a “dude” than a bad word. Technically, it can be a bad word but I can’t explain its origin (I can’t, and I don’t really bother to either).

He’s in 4th grade so he is supposed to be learning a lot of curse words and practicing them but he’s obviously a peacekeeper neglecting his age-appropriate duty. I don’t know what kind of person he will become. I’m kind of looking forward to it though.