Accy fail means: An accy fail is the specific, crushing moment when your accumulator bet collapses - usually on the last leg, in the last minute, from a team that had no business equalising. It's a rite of passage. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.