"piss-taker" means british slang for a person who takes the piss — that is, someone who mocks, teases, or tests the limits of what others will put up with. 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.