Slug is an interesting word.
It starts with the animal:
and then two different attributes of the animal diverge into different paths of meaning.
The first is the shape of the thing, which gives us a word for a bullet:
and also for small pieces of the lead type used for printing, and (in Canada) tokens for slot machines. It gives us the word which describes a small amount of powerfully alcoholic drink, too.
If you slug someone then you're hitting them with the force of a bullet (or perhaps a drink); but a person who's a slug isn't rushing round like Superman, and this is because the animal slug doesn't itself exactly frisk and scamper as it goes about its daily tasks. For this reason a human slug don't slug people, because a slug means lazy person. (A sluggard can hardly be bothered to get up because he's originally a slug abed.)
To end on something regally lovely, here's a sea slug:
I hope you spot your slug before you tread on it!
Spot the Frippet: slug. This word probably came from Scandinavia, and in English it first of all meant a slow person or animal.