Pharaoh's heart was hardened

Here "heart" refers to Pharaoh. His stubborn attitude is spoken of as if his heart was hard. See how you translated this in Exodus 7:13. Alternate translation: "Pharaoh became more defiant" (See: rc://en/ta/man/jit/figs-synecdoche and rc://en/ta/man/jit/figs-metaphor)