a person cannot change his basic, true nature
—I don’t know why I expect my Tammy to stop lying. A leopard can’t change its spots.

Jeremiah, a prophet to the Israelites, told the people God’s view of how bad their disobedience had become:

But there is little hope for you ever doing good,

you who are so accustomed to doing evil.

Can an Ethiopian change the color of his skin?

Can a leopard remove its spots? [Can the Ethiopian change his skin, or the leopard his spots?] (Jeremiah 13:23)

The implied answer to the question in this verse is “No,” and the meaning of the verse is this: The Ethiopian and the leopard cannot change how they look, and the evil people of Israel have made sin such a habit that they are in danger of not being able to do good.