"antirentism" means a historical american political movement and ideology from the 1840s, primarily in new york state, in which tenant farmers ("antirenters") resisted the feudal-style leasehold system imposed by large landed estates, particularly those of the van rensselaer family. It is one of many casual words teens pick up from friends, social media, music or sport, and on its own it carries no particular warning. If you hear your teen use it, it is fair to ask what they mean by it in their friend group, since meanings drift quickly. Asking out of curiosity rather than alarm tends to keep the conversation open and useful.