Activist justice means: A justice (usually referring to a member of a Supreme, High or Appellate court) who makes rulings based on personal political views or considerations rather than on the law, or who issues rulings. 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.