"back gray" means: A piece of dark, heavy cotton cloth that is run under another piece of cloth to be printed, serving to take up the color on the roller outsi.... 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.