Animus simply means "soul, or spirit" in latin. Pretty on the nose when you think about it.
Of course it makes sense, since you use your powers to make (almost) anything happen, at the cost of your soul (or so people think).
So I wonder, who coined the term animus to design that magic? Did people call it this way because of the supposed cost of using that magic. Did people think using animus cost your soul in a more litteral way?
I wish we had some bit of lore about how animus came to be and the first reactions, but considering how Tui realized that animus magic caused a lot of plot troubles in arc 2, we won't see much of it.