punching bag

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A stuffed or inflated leather bag that is usually suspended so that it can be punched or kicked for practice or exercise.
• noun A target of frequent criticism or abuse.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device, consisting of a tough cloth bag over sand or other material, used to practice punching.
• noun A person serving as an object of abuse.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an inflated ball or bag that is suspended and punched for training in boxing
• noun a person on whom another person vents their anger