"X-pac heat" is when fans boo a wrestler because they dislike the wrestler personally as opposed to the character he or she plays in the ring. In wrestling slang, heat refers to negative crowd reactions.
Wrestlers who play villainous characters, who are called heels within the industry, generate heat by insulting fans or cheating in the ring. X-pac was the ring name of wrestler Sean Waltman. Fans and wrestlers alike came to dislike Waltman due to WWE writers overexposing him despite lukewarm crowd reactions and his backstage political power. Fans chanted "X-pac sucks," at events where Waltman was not even present. X-pac heat now refers to wrestlers who are booed because the fans do not want to see them at all.