Kevlar means: (Fabric, cordage, etc made of) aramid fiber, an exceptionally strong, light, man-made fibre used to strengthen cables and sheet materials, e.g. in stab-resistant vests.. 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.