Weak and unhealthy because you have not been getting enough food, or not the right kind of food. It is worth seeing that second half clearly: undernourished is not only about an empty stomach. A child who eats a full bowl of white rice every day and almost nothing else can be undernourished — the quantity is there, the vitamins and protein are not. That is why doctors prefer this word to 'hungry': hungry is how you feel right now, undernourished is what months or years of poor eating have done to your body. It is a fairly formal, medical-sounding word; you meet it in news reports about famine, in health reports, in charity appeals. In everyday Taiwanese-student English you would more likely say 'he doesn't eat properly' or 'she isn't eating enough'. It describes people and animals, and it can also be used of soil or plants in gardening writing. You can be slightly undernourished or severely undernourished — it is a matter of degree, not a yes/no state. The related noun for the condition is 'malnutrition', and its adjective 'malnourished' is very close in meaning; 'malnourished' is the more common one in medical writing and can also cover the wrong balance of food, not just too little.

營養不良的（不只是吃太少，也包括吃得不均衡）