Here's what might be happening:
* Misheard or Misremembered: You might be thinking of a similar-sounding idiom or phrase. There are several idioms that deal with temperature and numbers, but none directly match "cold on nine."
* Specific Context: If you heard this phrase in a specific context, like a book, movie, or conversation, it's possible it's a unique expression within that particular setting.
* Regional Slang: It's possible that "cold on nine" is a slang expression used in a particular region or community.
To help me understand what you meant:
* Can you provide the context in which you heard this phrase?
* Do you have any other clues about its possible meaning?
With more information, I might be able to give you a more accurate answer!