Someone who is malnourished is ill or weak because their body has not had enough good food. The picture most people have is hunger, and news reports often mean that. Though actually, you can eat a lot and still be malnourished if the food has almost no protein, vitamins, or other things a body needs. Children who are malnourished may grow slowly; anyone may feel tired, catch illness easily, or look poorly in hair and skin. It is a fairly formal, medical-sounding word, used by doctors and aid workers. It describes the person (or animal). The problem itself is malnutrition. Being thin is not the same thing: a thin person may be perfectly well fed.