Description and usage
The footstool was a piece of furniture on which a person could rest his feet. It took the form of a low stool on which the feet were placed to keep them off the ground. Egyptian footstools, such as the one shown below actually depicted enemies the Egyptian king had defeated.
Translation

Except for JAS 2:3, the Greek word hupopodion occurs only as a figure in the New Testament. As a figure, it occurs in quotations from or allusions to Old Testament verses. Most translations consulted render it literally; for example, in LUK 20:43GNT has “until I put your enemies as a footstool under your feet.” NCV renders it nonfiguratively, saying “until I put your enemies under your control.” Similarly, at MAT 3:35 instead of the literal “or by the earth, for it is his footstool” (RSV), NCV has “using the name of the earth, because the earth belongs to God.” In JAS 2:3hupopodion refers to an actual footstool, when the host insults the poor man by telling him, “Sit under my footstool.” REB retains the reference to the footstool with “sit here on the floor by my footstool.” However, most translations choose to omit the footstool with something similar to NIV, which says “Sit on the floor by my feet.”