A couch potato is someone who is lazy or not very physically active, especially someone who spends a lot of time watching television and eating junk food.
This is a relatively new idiom that originated in America in the 1970s, and is now frequently used in the UK.
Examples of use:
1. News headline: 'Couch potato' children risking health.
2. My husband is turning into a couch potato. All he ever does is sit in front of the TV, eating pizza and drinking beer.
Image © Nevada Tumbleweed