In sports (both real and fantasy) a player with a high salary that underachieves or performs below expectations. This player ties up a considerable amount of salary under the salary cap, hence the name cap hog.
"Smith is only scoring 3 points a game. We need to get rid of him, he is just a cap hog at this point."
"I need to get Johnson off of my team, he is just a cap hog at this point."