It's an expression:
Nuclear heat/"X-Pac heat"
While other methods of heat try to draw disdain, "nuclear heat" (also known as "X-Pac heat", due to X-Pac becoming a notorious example of the concept) is when the fans legitimately dislike the character (and sometimes the person playing that character). This kind of heat can often be piled onto wrestlers who are viewed negatively, either for their booking, their character or even actions outside of the show. A current example of X-Pac heat would be Roman Reigns.
