Adam Clarke on Exodus 6:12

Uncircumcised lips? - The word ערל aral, which we translate uncircumcised, seems to signify any thing exuberant or superfluous. Had not Moses been remarkable for his excellent beauty, I should have thought the passage might be rendered protuberant lips; but as this sense cannot be admitted for the above reason, the word must refer to some natural impediment in his speech; and probably means a want of distinct and ready utterance, either occasioned by some defect in the organs of speech, or impaired knowledge of the Egyptian language after an absence of forty years. See Clarke's note on Exo 4:10.