A number of times, Moses repeats the he is "slow of speech" (Ex. 4:10) or has "uncircumcised lips." (Ex. 6:12 and 6:30, KJV; "faltering lips" in NIV) The Jewish midrash, or story, surrounding this goes as follows: as a young child, Moses tried to grab Pharaoh's crown. Pharaoh's advisors claimed this was a sign, and that he should kill the child. However, one of them asked that Pharaoh place before baby Moses two plates, one of burning coals and one of gold and precious jewels. If Moses reached for the jewels, it would mean he knew what he was doing in grasping for the crown and should be killed. If he reached for the coals, it would mean he was just a foolish child and could be permitted to live. Though Moses wanted to reach for the jewels, at the last moment God intervened and caused him to grasp the coals instead. As he burnt his hand, he put his fingers in his mouth and burnt his tongue as well. Ever since, he was slow of speech and tongue.
Though the general outline is the same, details of the story differ. Here are a few links to variations:
- Jewish Fairy Tales and Legends
- Chabad.org (a more condensed version)