Abreviation, to be well hung, have an abnormally large penis, it is usually split into three parts, Senior, Intermediate and Junior starting at the tip going to the base, Junior and Senior interchange but Intermediate is always in the middle. If you are FBS you bang a lot of birds, but don't like bragging about it.
Geoff likes boys and willies
Jones: FBS! dude, i got that Fat Boy Swag
Jones: dude help me out of this jacket im stuck (starts to sweat and is struggling to breathe)
Glenn: haha "FBS" (Fat Boy Struggle)
Jones: Thanks man! (sarcasm)
This term originates back to the summer of 2007. Originally an involuntary spasm when touched by a disapproving female, it has since been adopted by others as a formal way to communicate "that's really gross", or "that's fucked up."
It can be used by males and females and should not be overused. If a person is incapable of shaking the entire body in disgust, they are not allowed to say "aww, fbs", and are excluded from the fan club.
Jon: Yeah but he's having an fbs, he's clearly not interested
Jen: What a relief! I'm glad fbs exist!
NOTE: The above example is based on an extremely true story. I swear.