FJB is an acronym, which represents the phrase, "Fucking Japanese Bitch." As the wording suggests, FJB is used primarily in reference to Japanese women, or simply women native to Japan.
ME: That Japanese girl thinks she is so cute with her trendy clothes and fashion. I would consider impregnating her if she wasn't such a bitch.
HIM: What an FJB. Get these weirdos out my face.