Here "heart" refers to Pharaoh. His stubborn attitude is spoken of as if his heart was hard. See how you translated this phrase in [Exodus 9:12](../09/12.md). AT: "caused Pharaoh to became more defiant" (See: [[rc://en/ta/man/translate/figs-synecdoche]] and [[rc://en/ta/man/translate/figs-metaphor]])