"gaff mode" means: The relaxed, unguarded state a person enters when they are at home and fully comfortable — not performing for anyone, fully decompressed, do.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.