Words beyond literal meanings: What does the word couch potato mean?

I am tired of being used as everyone’s punching bag. When my husband can’t make a call to someone due to insufficient balance in his account, he shouts at me. When my mother-in-law can’t watch her favourite serial on TV because of power cuts, she vents her anger on me. Hereafter, I’ll not be anyone’s punching bag.

What is the meaning of the term punching bag? Literally, a punching bag or punchbag is a stuffed cylindrical leather bag that is hung on a rope. Boxers and other sports persons punch them as part of exercise. Figuratively, it means any person on whom anger is vented by another person in an unjustified manner. An alternative term for punching bag is ‘scapegoat’. If a person is made a scapegoat, they are blamed for the fault of another person. For example, some teachers are made a scapegoat for students’ poor performance in public exams. Here are examples of how ‘punching bag’ is used in sentences:
• Whenever he makes a mistake, he shouts at his wife and blames her. The voiceless woman is her husband’s punching bag.
• As the Congress party won only 19 of the 70 seats it contested in Bihar, it was severely criticised by its allies. Recently, Congress has been every party’s punching bag.
A few days ago, a friend asked me whether the term couch potato is an offensive term. A person who is described as a couch potato spends a great deal of time watching TV and is inactive. Any lazy person can be called a couch potato. The term has a negative connotation because nobody wants to be called a couch potato. The term was first used in the 1970s by a comic artist who drew lazy characters called couch potatoes. 
Is it an offensive term? Here is an interesting piece of news. According to the BBC, British farmers consider the term offensive. The farmers say that potatoes are healthy and are of the view that the expression portrays the vegetable as unhealthy.  They say that it is unfair to use the term ‘couch potato’ to describe someone who is lazy. They have held protests and demanded that the Oxford English Dictionary remove the term from the dictionary and use some other expression for idlers who waste their time watching TV but their demand has not been accepted by the publisher. This makes me raise the question whether the ‘potato’ is treated as a punching bag.  
• Eating potato is good for health but being a couch potato is not good for health.
“Everyone should be proud of who they are and where they come from because America is a big melting pot of diverse ethnicities” 
What does the term melting pot mean? It literally means a pot in which different materials are mixed and melted.  The other meaning of the expression is ‘a place where people or ideas of different kinds get mixed together.’ It denotes pluralism and conflation. It has a positive connotation as the melting together of different aspects results in producing something new is to be celebrated.
• Very few universities in India are a melting pot of diversity.
• Chennai is a cultural melting pot.