I could be wrong here, but I've seen it spelled "ba fangool", and been told it means "go f*** yourself". Someone else told me it means simply "f*** you"

It also comes up in GF I, during the big dish throwing fight between Carlo and Connie. I may have this backwards, but I think Carlo says it to Connie, and then she says "Oh, ba fangool you". It might have been the other way around; I'm not sure.