In baseball, a slugger is a player who hits the ball a very long way — the kind of batter who can send it out of the park. It is not only 'a good hitter'; the picture is power, not small neat hits. People also stretch it a little to anyone who hits hard, including boxers, but in everyday sports talk it is mainly a baseball word. From slug 'to hit hard' plus -er 'a person who does that.' Informal to ordinary sports language, not stiff or fancy.