The closing line to any fight or disagreement between a teenager and either his father/mother or (more commonly) his important replacement parental figure (in the right genre, between the hero and his mentor). This is either a loophole for someone who really is in the wrong and wants a reason to end the argument without defending himself, or a way for a Good-aligned (or standards-bearing) offspring to inform his Evil-aligned father [figure] that he has crossed the line and is no longer worthy of respect, admiration, or obedience. A twist that turns up the tension is "You're not his father."