punchbag
/ˈpʌntʃbæg/noun
1- Brit. a stuffed cylindrical bag suspended so it can be punched for exercise or training, especially by boxers〈英〉（尤指拳击选手锻炼或训练时使用的）吊袋。
1.1
- a person on whom another person vents their anger发泄怒气的对象, 出气筒。