Kebesh (H3534) refers to a footstool or step, specifically the step or footboard on which one rests feet. It is closely related to kabash (to subdue, tread down) — the footstool being the place of conquered enemies under royal feet.
God declares: 'Heaven is my throne, and the earth is my footstool' (Isaiah 66:1). Psalm 110:1 — the most quoted OT verse in the NT — envisions enemies made a footstool for the Messianic King. The writer of Hebrews applies this to Christ's completed atonement: He sits enthroned, waiting until His enemies are made His footstool (Hebrews 10:12–13). Full sovereignty includes total subjugation.