Acoustic fail means: When someone's attempt at genuine, earnest sincerity goes horribly wrong in a way that produces maximum secondhand embarrassment - because they were fully committed and completely unaware of how it. 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.