This root is also found in words like:
* Foundry: A place where metal is melted and cast into shapes.
* Infusion: The process of pouring a liquid into something to extract its properties.
* Confound: To mix up or confuse, as if pouring things together randomly.
So, "fusion" literally means "the act of pouring together," which is a fitting description for the process of combining two things into one.