“dog”是“狗”，“two tails”是“两条尾巴”，如果小狗长了两条尾巴，同时摇动，表达出来的快乐，“be like a dog with two tails”指一个人心情特别愉快，也就是“非常高兴”。

【例句】
He was like a dog with two tails when he got some pocket money.
当他拿到零花钱，他高兴极了。

She was like a dog with two tails when she saw the beautiful dress.
看见那条漂亮的裙子，她非常高兴。