Exodus 6:12
And Moses spake before the LORD, saying, Behold, the children of Israel have not hearkened unto me; how then shall Pharaoh hear me, who am of uncircumcised lips? And Moses spake before the LORD, saying, Behold, the children of Israel have not hearkened unto me; how then shall Pharaoh hear me, who am of uncircumcised lips?
Moses spoke while somewhat discouraged and despondent because Israel had refused to listen to his message of deliverance. He therefore doubted that Pharaoh would listen to him. He reasoned from the lesser to the greater, that if God's own circumcised people would not hearken to him, then Pharaoh, who is uncircumcised and an idolater, would not hear him either. By calling himself of uncircumcised lips he expressed his sense of inadequacy as a speaker. God answered by appointing Moses as a god to Pharaoh and Aaron as Moses' prophet to speak all He commanded them, declaring that Pharaoh would not listen and that He would bring Israel out through great judgments.