There are a variety of synonyms that can be used to describe poorly carbonated juice. Some options include flat juice, uncarbonated juice, still juice, or juice without fizz. Another option is to describe the juice as lacking in effervescence, meaning it has a lack of bubbles or gas. Additionally, it could be described as not having a lively carbonation or as being devoid of the typically refreshing fizziness that is associated with carbonated beverages. No matter how you choose to describe it, the commonality among these synonyms is that they all convey the same idea of a juice that is lacking in carbonation.