Gastropods: When we talk about slugs, we need to talk about snails as well. Both belong to the class gastropods, a term invented by Georges Cuvier in 1795, from the Ancient Greek γαστήρ + πούς (“stomach + foot”) and they form with mussels and octopuses the group of molluscs. All of them are basically squishy beings, thus was named molluscus (mollis, soft). The taxonomy of the family is constantly being revised. The concept of slug is very vague and it is hard to compare from one language to another.
The pink group are languages that uses a specific word for slug, either the greek root leimax or the slavic root slimak, both are derived form Proto-Indo-European slehym. Some other languages do not have a word for it, but it is formed by adding naked to snail. It is not an obscene remark, but a mere observation: a slug is a snail without shell. Other groups do not share an etymological common origen but a semantic one. The yellow group are words that describe the animal as some sort of slippery substance. Spanish babosa means “drooling”, Turkish, Russian and Lithuanian. The last group are the greens in the map, the crawlings.