From an unused root meaning to be clear, i.e. Conspicuous; the forehead (as open and prominent) -- brow, forehead, + impudent.
H4696. metsach
מֵ֫צַח noun masculineExodus 3:8 brow, forehead; — ׳מ absolute Ezekiel 3:7, construct Exodus 28:28; Jeremiah 3:3; suffix מִצְחֲךָ Ezekiel 3:8; Isaiah 48:4, מִצְחֶ֑וֺ Ezekiel 3:9; מִצְחוֺ Exodus 28:38 4t.; מִצְחָם 3:8; plural construct מִצְחוֺת Ezekiel 9:4; — brow, forehead, struck by stone from David's sling 1 Samuel 17:49 (twice in verse); Aaron's forehead Exodus 28:38 (twice in verse); place where leprosy appears 2 Chronicles 26:19-20, place for a distinguishing mark or cross Ezekiel 9:4; ׳חִזְקֵימֿ 3:7 the house of Israel are stout of forehead (i.e. obstinate; "" קְשֵׁילֵֿב), compare 3:8; so נְחוּשָׁה מִצְהֲךָ Isaiah 48:4 thy brow (is) brass ("" עָרְמֶּ֑ךָ בַּרְזֶל גִּיד); also of boldness, firmness of prophet 48:8, and 48:9 (but dele ᵐ5 Co); מ לָךְ הָיָה ׳זוֺנָה אִשָּׁה Jeremiah 3:3, i.e. thou wast shameless ("" הִכָּלֵם מֵאַנְתְּ).