He kept looking at his watch anxiously, worried he would miss the train.

anxiously 是形容词 anxious 的副词形式，anxious 的意思是"焦虑的、担心的"，加上 -ly 变成副词后表示"焦虑地、不安地"。 这里 anxiously 修饰动词 looking，描述他看手表时的焦虑状态。