Exodus 6:12

And Moses spake before the Lord
Who appeared in a visible form, and had spoke to him with an articulate voice, and before whom Moses stood, and made the following reply:

saying, behold, the children of Israel have not hearkened unto me;
even though he brought a comfortable message to them from the Lord, and delivered many gracious promises of his to them, assuring them of deliverance out of Egypt, and of their possession of the land of Canaan:

how then shall Pharaoh hear me?
making a demand upon him to part with a people, from whose labour he receives so much advantage, and has such an addition to his revenues, and who is a mighty king, and haughty monarch. And this is further enforced from his own weakness and unfitness to speak to Pharaoh:

who [am] of uncircumcised lips?
had an impediment in his speech, could not speak freely and readily, but with difficulty; perhaps stammered, and so uttered superfluous syllables, repeated them before he could fully pronounce what he aimed at; or in other words, he was not eloquent, which was his old objection, and had been fully answered before: and by this it appears that there was no alteration in the speech of Moses since God spoke with him at Mount Horeb. Some think Moses expected to have had this impediment removed, and tacitly hints at it here, not being so well satisfied with Aaron's being joined with him as his mouth and spokesman, which seemed to carry in it some reflection upon him.