In my house, we often ask which words come first–the animals, or the people who share their traits. Like slug/sluggish, sloth/slothful etc.
Well, in the case of slug, the trait definitely came before the critter! It comes from the Scandinavian word slugje, which means “a slow, heavy person.” It’s been in English since the early 15th century. Interestingly, it wasn’t given as a name to a shell-less snail until 1704!
Etymologists aren’t quite sure where the next meaning of slug–a lead bit–came from. Perhaps because of how heavy lead is? No one’s quite sure. But it’s from this secondary meaning that we get the “bullet” meaning. This may have led directly to the meaning of “a hard blow or punch.” The meaning of “swallow” is likely influenced by the Irish slog, which means swallow.