Sloths are slow.
Sloth, meaning laziness, is also one of the Seven Deadly Sins in Christianity.
Unsurprisingly enough, the sin came first, and the animal was named after it, because it seems so lazy. They’re not actually lazy of course, just slow. Though some individual sloths probably are lazy, but it’d obviously be unfair to tar them all with the same brush.
It’s a funny word, sloth, and not one we really use beyond these two contexts. Where does it come from?
The answer is actually surprisingly straightforward, and you might be able to figure it out if you look at the word. Sloth is a now-obsolete noun form of the word slow. Originally it was slowth. Adding the suffix -th was once a common way to form nouns of action, quality, or state from adjectives. We can still see a few examples in English: warmth, strength, length, width, and so on. Slowth gradually became sloth, probably because the former is a bit awkward to say. And because sloth isn’t clearly linked to slow, its meaning drifted a little to mean lazy. And because people didn’t realise that it was the noun form of slow, slowness came into being as the noun form instead (and it does sound better than slowth, doesn’t it?).
So even though sloths got their name because they seemed lazy, it’s kind of appropriate that their name is sloth, as they are just slow, harking back to that original meaning.
Also, they’re very cute.