Halftime means: The halftime show, the primary light entertainment of a game, after the second quarter, when players can physically recover, coaches can give players a pep talk, bets can be doubled, etc.. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. It is sporting vocabulary and harmless in itself. 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.