In heraldry, "fess" refers to a horizontal band that divides a shield into three parts.
If you are looking for a French word that has a similar meaning to "fess" in a different context, you might be thinking of:
* "fesse" (plural: "fesses") which means "buttocks".
* "faire face" which means "to face" or "to confront".
If you can provide more context about how you encountered the word "fess", I might be able to give you a more specific answer.