Here "hearts" refers to the Egyptians themselves.Their stubborn attitude is spoken of as if their hearts were hard. See how you translated a similar phrase in [Exodus 9:12](../09/12.md). Alternate translation: "I will cause the Egyptians to became more defiant" (See: [[rc://en/ta/man/translate/figs-synecdoche]] and [[rc://en/ta/man/translate/figs-metaphor]])