"pyx" means: A box used in a mint as a place to deposit sample coins intended to have the fineness of their metal and their weight tested before the coin.... 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.