From an unused root meaning to stamp upon; a foot stool -- (foot-)stool.
H1916. hadom
הֲדֹם noun masculine stool, footstool, always joined with רַגְלַיִם; only in poetry and late writings, (properly something cast down, low) — ׳ה absolute Psalm 110:1 (or construct before לְ, compare Ges§ 130, 1); construct Lamentations 2:1+ 4t. — footstool, never literal, usually of ׳י Isaiah 66:1 ה וְהָאָרֶץ כִּסְאִי ׳הַשָּׁמַיִם רַגְלַי; elsewhere with reference to sanctuary ׳ה רַגְלָיו (compare רַגְלַי מְקוֺם 60:13) Lamentations 2:1, or perhaps of ark (as place on which God rests) Psalm 99:5; 132:7, אֱלֹהֵינוּ בְּרִיתיֿ לַאֲרוֺן מְנוּחָה ׳בֵּית ׳וְלַה רַגְלֵי 1 Chronicles 28:2; of conquest of enemies of Messianic king by Yahweh's agency לְרַגְלֶיךָ הֲדֹם אֹיְבֶיךָ אָשִׁית Psalm 110:1.