The wagtail was hopping cheerfully on the grass, its tail constantly wagging up and down.

wagtail 是由 wag（摇摆）和 tail（尾巴）组成的复合词，字面意思是"摇摆尾巴的鸟"。 wag 是动词，意思是"摇摆、摆动"。 tail 是名词，指"尾巴"。 wagtail 特指鹡鸰科的小型鸟类，这类鸟的特征就是会不停地上下摆动尾巴。