What does the prefix fid mean?

The prefix "fid" doesn't exist as a standalone prefix in English. It's likely you're thinking of a combination of letters that appear in various words, but don't form a consistent prefix.

For example:

* "Fid" might be part of a larger prefix: Like "fidu" in "fiduciary," which means "holding something in trust for another."

* "Fid" might be part of a word root: Like "fidel" in "fidelity," which means "faithfulness."

If you can give me the full word you're thinking of, I can help you understand its meaning and origin better!