Informal. When something is full-on, it is as strong, loud, busy, or complete as it can be — nothing held back. A full-on party is wild, not a quiet cup of tea; a full-on argument is a real fight; full-on summer heat means the hottest kind. You can also use it like an adverb: they went full-on with the decorations; she is full-on in exam week. Younger speakers use it a lot, and it is especially at home in British and Australian English, though Americans say it too now. It belongs in chat, messages, and casual stories, not in a formal report. People stretch it: “full-on mode” just means they are not holding anything back. Though actually, it does not mean a machine is switched fully on; it means extreme.