Nonfat food has had the fat taken out, or is made with none of the fat left in. You see it most on milk, yogurt, and milk powder: nonfat milk is the kind with the cream removed (often called skim milk in some places). The word is packing-label English, not a word for a person's body. It tells you about the food, not about whether someone is thin. Fat-free on a label is the same idea; low-fat still has some fat. Though actually, taking the fat out changes taste and mouth-feel, and makers sometimes add sugar or thickener to make up for it — nonfat does not mean 'nothing added'.