Mabo refers to Eddie Mabo, the Australian Indigenous land-rights activist whose 1992 court case overturned the doctrine of terra nullius. Not concerning, just historical and political vocabulary. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.