nickleback

a defensive back in American football who is the fifth defensive back, often used in passing situations

A nickelback is a player on the defensive team in American football who comes into the game when the other team is likely to pass the ball. The nickelback is added as a fifth defensive player, usually to help cover an extra wide receiver. Their main job is to stop passes and prevent the other team from making big plays. They are similar to cornerbacks but are used in special situations when the offense has more players trying to catch the ball. Nickelbacks need to be fast and good at defending against passes.