In Dante's Inferno, the ninth circle of hell is the centre where the worst punishments are found, so it is used idiomatically for something that couldn't get worse.
An ninth idiom dictionary is a great resource for writers, students, and anyone looking to expand their vocabulary. It contains a list of words with similar meanings with ninth, allowing users to choose the best word for their specific context.
Dictionary of similar words, Different wording, Synonyms, Idioms for Idiom, Proverb ninth