What is the prefix for Hy?

There isn't a specific prefix for "Hy" in the English language. Prefixes are typically used to modify the meaning of a word, and "Hy" itself doesn't have a clear meaning on its own.

It's possible you're thinking of a specific word or phrase that starts with "Hy". If you can provide more context, I might be able to help you find the prefix you're looking for!