She doesn't have to be well read. Maybe she heard the word on a YouTube video, or read it in a blog comment, and liked the word so uses it. I've found a lot of kids do this. Thanks to that big truck known as the internet, there's so much access to, well... everything that kids today know a lot more random things / words / phrases / etc than you can shake a stick at, things you wouldn't expect a kid their age / from their cultural background / whatever to know. It all depends on if it fits your character and story.