FBS, or Fat Bitch Syndrome, is suffered by very fat, usually disgusting, women.
Instead of becoming shy, docile, and hermetic like most fat women, the woman with FBS has a very different, enigmatic reaction. Her response to being so grossly obese men that never look at her was to become even MORE full of herself than a normal girl. She believes that the reason men avoid her is that "they can't handle this!"
Though the fat bitch is hideously blubberous, she will often still believe that she is, in fact, attractive. This is often because she is still hit on by rednecks, white trash, and hobos, usually at truck stops. You will see frequently her wearing "Babydoll" t-shirts in sizes 3 or 4 below what she actually needs, leading to the DJRP, the Dangling Jelly Rolls Phenomenon.
If you know a woman suffering FBS, don't try to explain the issue to her. You will only be met with exclamations of "Nuh-uh, nuuuuh-uh, boy, you did NOT go there." Avoid at all costs.
Everyone can think of their own example of a woman suffering from Fat Bitch Syndrome (FBS). If you can't, you either are one, or you're white trash who's too busy feeding the egos of FBS sufferers everywhere to notice.