It's best to avoid using this word altogether and find more appropriate ways to express yourself.
If you're looking for a less offensive way to refer to the buttocks, you could consider using:
* "ffond" (bottom)
* "cw*l" (arse) - while this word is considered more vulgar than "ffond", it's less offensive than "ffag".
Remember, it's always best to be respectful of language and avoid using offensive terms.