noun - a person that is extroverted to the point of nausea, often to overcompensate for poor looks or a large frame/body type. Due to the psychological affect of being overweight, the "fatty" tries to overcompensate this negative with an overly obnoxious personality and attitude.
adj - of or exhibiting the "fatty" lifestyle.
Jesus, Pam needs to shut up already. She's such a fatty.