"Naughty Nineties" means: A British nickname for the 1890s, a decade widely remembered for its social liberalism, artistic decadence, and moral permissiveness relativ.... 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.