Acoustic core means: An aesthetic and attitude defined by unironic earnestness, wholesome vibes, and the complete absence of cool detachment - think flannel, folk music, heartfelt journal entries, and an almost. 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.