The incantation Anorexianervosa is a play on the disease 'anorexia nervosa' - an eating disorder most prominent in young women who are conscious about their weight. People with this disease refuse to eat. This spell allows the caster to make the target as thin asa rail. The target, however, does not become anorexic nor suffer the ill effects of the disease such as malnutrition. It's an excellant weight loss spell.
Ermine used this spell while breaking into Fantastic! in order to make herself look more like Jane so she could rescue J.G.