Tennis-core is a fashion aesthetic of tennis whites and preppy sporty styling. Same as tenniscore but with the hyphen. Not concerning, just fashion. 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.