A psychological condition affecting women who used to be overweight and made a "lifestyle change," causing them to lose a lot of weight.
This last one is especially troubling, since sufferers of this syndrome are simultaneously invalidating their own hard work by making it seem as though anyone can lose weight easily at any time and being horrible to a group of people they used to belong to. Many former fat girls know exactly why there is a lot of work to getting in shape, they have had to deal with psychological issues about food, self esteem, and exercise. They have had people call them names or make assumptions about what they could or could not do. They have gone to a store and tried on 15 pieces of clothing, found nothing that fit right, and gone home in tears. They know how much fat shaming hurts, and they do it any way.
Former fat girl syndrome can affect men as well as women, although for obvious reasons it is more common among women. The syndrome often results in internet trolling. As of now, there is no known cure, but time, patience, and reminding these people that there is more to life than attractiveness and more to attractiveness than weight are all things that can help treat the syndrome.