Here "heart" refers to Pharaoh. His stubborn attitude is spoken of as if his heart was hard. See how you translated a similar phrase in [Exodus 7:13](../07/11.md). AT: "Yahweh caused Pharaoh to became more defiant"(See: [[rc://en/ta/man/translate/figs-synecdoche]] and [[rc://en/ta/man/translate/figs-metaphor]])