Quoting Commentary for Song of Songs 4:3

And Rashi brings a parable about a pearl that fell among sand, etc. He compares the house of Esau to sand and the house of Jacob to the pearl, following the explanation in Leviticus Rabbah (4:6): Esau had six souls in his household, yet the verse refers to them as “souls” [plural], because they worshipped many gods. Jacob had seventy souls in his household, yet the verse refers to them as “soul” [singular], because they worshipped one God. End quote. And because they [Esau’s household] worshipped many gods, strife was constantly present among them, and they lacked unity due to their multiple religions, and they do not unite and connect like this sand that doesn’t bind together. But Israel, because they worship one God, peace mediates between them, and they are like pearls strung on a thread, attached to one another through the thread that connects them, as it says like a scarlet thread are your lips (Song of Songs 4:3) — lips of knowledge and Torah that increase peace in the world, like this thread that strings the pearls together.

YISA MIDABROTHECHA.’ The word yisa is similar to: Thou shall not ‘thisa’ (take) the Name of the Eternal thy G-d in vain;⁵⁵Exodus 20:7. and nor ‘esa’ do I take their names upon my lips;⁵⁶Psalms 16:4. ‘yisa’ (he shall swear) in that day, saying.⁵⁷Isaiah 3:7. And the word midabrothecha is like the expression ‘umidbareich’ (and thy mouth) is comely.⁵⁸Song of Songs 4:3. He thus states [in the verse before us] that Israel will take Your words upon his lips and meditate upon them always.”

this the side of the height of the face, as in Song of Songs 4:3 "Your brow behind your veil [Gleams] like a pomegranate split open." and its called this because of the lack of flesh on it as in Bresheit 41:19 "emaciated."