The imagery is based on the idea of being so poor that you have only rocks to your name.
Here are some examples of how it's used:
* "He's been struggling for years and is now rock poor."
* "After losing everything in the fire, they were left rock poor."
It's important to note that this expression is somewhat outdated and might be considered offensive in some contexts. It's best to use more formal terms like "impoverished" or "financially destitute" if you're writing or speaking in a serious setting.
