Thin privilege is not having your body size quite seriously referred to as “gross” by someone who’s supposed to be mature and professional. (Note: I’m not aware of medical professionals referring to very thin people as “grossly underweight”, if it does happen it’s rare.) Thin privilege is also the concept of upper limits on BMI and waist measurement for non-emergency surgery that is not related to weight loss, whereas there are no minimum weight limits mentioned for any kind of treatment. (Other note: I am very aware that BMI and waist size are completely useless as measures of health.)