a footstool: the low stool under the feet of someone seated on a chair or throne, and so whatever is placed beneath a ruler's feet
The word is put together from a prefix meaning "under" and the ordinary word for foot (πούς), so it names simply the thing that goes under the feet. In the ancient world a seat of honor came with one: a chair or a throne stood raised, and a low stool kept the feet of the person seated off the floor. James 2:3 is the only place in the New Testament where the word names a real piece of furniture in a real room, and even there the point is rank, since the poor visitor is waved down to the floor beside the rich man's footstool.
The other six uses all describe what lies under someone's feet, and both pictures come straight out of the Hebrew Scriptures, where the matching Hebrew word for a footstool is הֲדֹם:
- Isaiah 66:1 sets heaven as God's throne and the earth as the stool under his feet, which is a way of saying that no temple anyone builds could ever hold him (Matthew 5:35, Acts 7:49).
- Psalm 110:1 promises a king that his enemies will be put where his feet rest, which was the standard picture of a war completely won (Luke 20:43, Acts 2:35, Hebrews 1:13, Hebrews 10:13).
Both pictures lean on the same piece of furniture, so they belong together. A throne (θρόνος) is where authority sits, and the footstool is the low edge of that same seat, so whatever ends up on it has been placed beneath the one enthroned. That is also why the Greek almost always says "a footstool of his feet" rather than just "a footstool": the interest is not in the object but in whose feet are resting on it.