Perhaps from an unused root probably meaning to be black; a coal, whether charred or live -- coals.
Englishman's Concordance
1) coal, charcoal, ember Part of Speech: noun masculine A Related Word by BDB/Strong’s Number: perhaps from an unused root probably meaning to be black Same Word by TWOT Number: 1760a
Brown-Driver-Briggs
מֶּחָםnoun [masculine]coal (collective); absolute ׳פ, as inflammable, לְגֶחָלִים׳פProverbs 26:21 (charcoal for embers, Toy); of glowing coal Isaiah 44:12, ׳אֵשׁ פIsaiah 54:16; Psalm 11:6 for ᵑ0מַּחִ֑ים אֵשׁ read מַּחַם אֵשׁ (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 מָּחַתdig, hollow out, also diminish).
Proverbs 26:21 HEB: פֶּחָ֣ם לְ֭גֶחָלִים וְעֵצִ֣ים NAS:[Like] charcoal to hot embers and wood KJV:[As] coals [are] to burning coals, INT:charcoal to hot and wood
Isaiah 44:12 HEB:מַֽעֲצָ֔ד וּפָעַל֙ בַּפֶּחָ֔ם וּבַמַּקָּב֖וֹת יִצְּרֵ֑הוּ NAS: his work over the coals, fashioning KJV: both worketh in the coals, and fashioneth INT: A cutting and does the coals hammers fashioning
Isaiah 54:16 HEB:נֹפֵ֙חַ֙ בְּאֵ֣שׁ פֶּחָ֔ם וּמוֹצִ֥יא כְלִ֖י NAS: the fire of coals And brings KJV: that bloweth the coals in the fire, INT: blows the fire of coals and brings A weapon