The phrase you're referring to seems to echo a biblical verse that speaks about the nature of change and human characteristics, which may be linked to Jeremiah 13:23. This verse states:
"Can an Ethiopian change his skin or a leopard its spots? Then may you also do good who are accustomed to doing evil." (Jeremiah 13:23, NIV)
In this verse, the reference to an Ethiopian (often connected to Cushitic people in some interpretations) and a leopard underscores the idea of inherent characteristics that are difficult or impossible to change. The verse is primarily about the human condition and the nature of moral behavior.