Exodus 6:12 - Matthew Poole's English Annotations on the Holy Bible

i.e. Of polluted lips. Uncircumcision being a great defect and blemish, whereby men were rendered profane, contemptible, and unfit for many services and privileges, may note any defect, whether moral, and of the spirit, or natural, and of the body. So here it notes Moses's inability to clothe God's commands in such words as might prevail with Pharaoh. But this was a great weakness of faith, as if God could not effect his purpose, because the instrument was unfit.