What does the prefix sub maen?

The prefix "sub" means "under" or "below." It's a Latin prefix that is used in a lot of English words.

"Maen" isn't a recognized prefix or word root in English. It's possible you're misremembering or misspelling a word. Could you provide more context or the full word you're thinking of?

For example, if you're thinking of "submarine," then "sub" is the prefix and "marine" refers to the sea.