There are many meanings for the word slug. You would have to look in a dictionary to see them all. But, the most common meanings might be: 1. To hit something. He got in a fight and slugged his friend. 2. An invertebrate that looks like a snail without a shell. The slugs are ruining the garden. 3. A drink, usually of liquor. He had a slug of whiskey before going to bed.

There are many meanings in the dictionary. I think that the following two are used more often. A small amount of a strong alcoholic drink. EX He took another slug of whisky. It also means to hit very hard and solidly.