The story
From the sport of boxing, referring to fighter who is too stunned to fight effectively, but who does not collapse to the canvas.
Etymology from Wiktionary, CC BY-SA 4.0.
out on one's feet
Today it means: Standing erect but not consciously aware of one's surroundings, or only minimally aware, and having little or no ability to control one's bodily actions, as a result of physical injury or exhaustion. Full definition →