Albert Barnes Commentary Exodus 6:12

"And Moses spake before Jehovah, saying, Behold, the children of Israel have not hearkened unto me; how then shall Pharaoh hear me, who am of uncircumcised lips?" — Exodus 6:12 (ASV)

Uncircumcised lips - An uncircumcised ear is one that does not hear clearly; an uncircumcised heart one slow to receive and understand warnings; uncircumcised lips, which cannot speak fluently. The recurrence of the hesitation of Moses is natural; great as was the former trial this was far more severe; yet his words always imply fear of failure, not of personal danger .