The word slug has several meanings:

1. A slug is a soft-bodied creature that looks like a snail without its shell. Slugs leave silvery slime trails as they crawl slowly across garden paths and leaves. Gardeners often find them eating lettuce and other plants at night. While they might seem gross, slugs play an important role in breaking down dead leaves and returning nutrients to the soil.
2. To slug someone means to hit them with a hard punch, usually with a closed fist. When two rivals slug it out, they're fighting physically. Baseball fans also use this word differently: a power hitter who slugs home runs is hitting the ball with tremendous force. A slugger is a player known for hitting powerful home runs.
3. A slug can mean a slow, lazy person. If your brother is still slugging around in his pajamas at noon on Saturday, he's moving slowly and not accomplishing much. The word sluggish describes this same slow, low-energy feeling, like how you might feel on a hot afternoon or when you haven't gotten enough sleep.
4. A slug is also an old word for a fake coin used illegally in place of real money in vending machines or parking meters.