Thou hast put all things in subjection under his feet. For in that he put all in subjection under him, he left nothing that is not put under him. But now we see not yet all things put under him. Hebrews 2:8
In ancient times, the king’s throne was elevated so that everyone who visited him had to bow down before him, sometimes even kissing his feet. His subjects were accordingly said to be “under his feet.” The feet were considered the lowliest part of the body, so to be under someone’s feet was to have lower status than him.
(NavPress Bible study, Hebrews, NavPress, Colorado Springs, CO: 1989, pg 35)