Another interpretation: “As I have hardened his heart” – Rabbi Yoḥanan said: From here there is an opening for the heretics to say that he had no [opportunity] to repent, as it is stated: “As I have hardened his heart.” Rabbi Shimon ben Lakish said to him: Let the mouths of the heretics be sealed; rather, “to the scorners He scorns” (Proverbs 3:34). The Holy One blessed be He forewarns a person the first time, the second, and the third, and if he does not relent, He locks his heart from repentance in order to exact retribution for his sins.
The same is true regarding the wicked Pharaoh. Once the Holy One blessed be He sent [him warnings] five times, and he disregarded His words, the Holy One blessed be He said to him: You were stubborn and hardened your heart; behold. I will add impurity to your impurity. That is: “As I have hardened [hikhbadti] his heart.”
What is hikhbadti? The Holy One blessed be He rendered his heart like this liver, which, when it is cooked twice, becomes hard. Thus, Pharaoh’s heart became like that liver, and he did not accept the words of the Holy One, blessed be He. That is: “As I have hardened his heart.”