Meaning:

A person who spends a lot of idle time sitting and watching television.

If a person is a couch potato, he/she is not active and spends most of his/her time sitting and watching TV.
He/she doesn't do anything productive.

Example:

School children don't play outside as much as before. Many of them are overweight couch potatoes.

Don't be a couch potato! You're just wasting your time.

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