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1.
a person who takes the blame when something goes wrong, a scapegoat
An unpopular politician is likely to become a punching bag.
by The Return of Light Joker March 31, 2009
 
2.
PUN-ching Bag (v) - The act of lifting the penis and inflicting pugilistic damage to the testicle area.
Alex barely recovered after Paige made a punching bag out of him.
by mr & jb September 06, 2006
 
3.
noun. When engaging in sexual activity with a particularly flexible girl (gymnast, ballerina, etc.), lift the girl's hips and push them directly over her face, so that her knees are alongside both sides of her head. By then penetrating her vaginally from the other side, there will be unavoidable scrotal slappage to the face. In this way, the testicles are like small fists punching a punching bag, and her face is like said punching bag. When performed at a rapid tempo, can be referred to as the speed bag. Note, this move becomes much easier to pull off when the female has great flexibility or the male has really droopy balls.
After finding out that the girl sitting next to him at the bar was a yoga instructor, Franklin's first thought was that he would take her home and have a shot at the punching bag.

OR

As Bonnie was being slapped repeatedly in the face by Herman's testicles, she protested, "Hey! Easy there! What do I look like? A punching bag?!" Herman responded, "Bingo."
by Steve^O March 18, 2008