Make off without payment is a legal phrase for leaving a service (restaurant, taxi, hotel) without paying. UK criminal offence under the Theft Act 1978. Not concerning vocabulary; concerning behaviour. 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.