"adult fail" means: An adult fail is a moment when the basic responsibilities of grown-up life defeat you in a way that's too relatable not to share. Forgetting.... 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.