Not given enough food over a long stretch of time, so that it shows on the body — thin, weak, small for their age. You use it about people and very often about animals: an underfed dog, underfed cattle, underfed children in a poor district. The important part is that it points a finger at someone else. Being underfed means someone else was supposed to feed you and did not do it well enough — parents, an owner, a government, a school. That is what separates it from 'hungry', which is just a feeling you have right now and passes with lunch, and from 'thin', which may be nothing more than the body a person happens to have. It also describes a lasting state, not one skipped meal. People do use it lightly and jokingly about themselves — 'you're not exactly underfed' when someone complains about their weight — but the plain meaning is a serious one.

營養不良、長期吃不飽（餵養不足）。常用在小孩、動物身上，帶有「照顧的人沒做好」的意味。