Deliverance...eventually (Exodus 5:1-6:13)

Contrary to the movies, Moses’s return was initially met with a positive response (4:31) but that quickly changed when Pharaoh started putting the screws to the Hebrews. When Moses goes to YHWH to ask why he would do things this way, YHWH replies with a series of promises predicated upon his nature, beginning with “I am YHWH” (6:1–9). Moses’s objection (6:12) is a weird one. What does “uncircumcised lips” even mean?