Perhaps from an unused root probably meaning to be black; a coal, whether charred or live -- coals.
H6352. pecham
מֶּחָם noun [masculine] coal (collective); absolute ׳פ, as inflammable, ׳פ לְגֶחָלִים Proverbs 26:21 (charcoal for embers, Toy); of glowing coal Isaiah 44:12, פ ׳אֵשׁ 54:16; Psalm 11:6 for אֵשׁ מַּחִ֑ים ᵑ0 read אֵשׁ מַּחַם (or מַּחֲמֵי) coals of fire ("" גָּפְרִית), so Ew and others
פחת (√ of following; compare Arabic cut off, VII. be perforated (of roof); Syriac Pa`el pierce, break through, pit, chasm; Late Hebrew מָּחַת dig, hollow out, also diminish).