Cooked, but not cooked enough. The heat did not go far enough in, so the middle is still soft, wet, or in the case of meat still red and cool. You use it about food that came out of a kitchen wrong: underdone chicken, underdone potatoes, an underdone cake that sinks in the middle. It is the natural opposite of 'overdone'. There is one important thing to notice about how people really say it. Nobody uses 'underdone' as praise. If a person likes their steak red, they ask for it 'rare' — that is a choice, and it sounds good. 'Underdone' means the cook made a mistake, and it is what you say when you send the plate back. So the same red steak is 'rare' when you wanted it and 'underdone' when you did not. Outside food, you sometimes hear it about work that was not developed enough — 'the ending of the film felt underdone' — meaning it needed more time and care. That use is not common in everyday speech, but it is easy to understand when you meet it.

沒煮熟、還沒熟透（食物中心還是生的）。注意：牛排的「三分熟」是 rare，那是客人自己要的；underdone 帶有「廚師沒做好」的抱怨語氣。