To unhorse someone means to knock or throw them off a horse. In medieval times, knights tried to unhorse their opponents during jousting tournaments by striking them with long poles called lances. If a knight succeeded in unhorsing his rival, he won the match and earned glory in front of the watching crowd.

A soldier who could unhorse enemy cavalry in battle gained a huge advantage, since a knight in heavy armor struggled to fight effectively once knocked to the ground. Today, you might read about unhorsing in historical novels or fantasy stories featuring mounted combat.

People sometimes use unhorse figuratively to mean removing someone from a position of power or authority, like when a challenger unseats a reigning champion. If a new competitor unhorses the defending spelling bee champion, they've knocked them off their metaphorical perch, just as surely as a medieval knight knocked an opponent off an actual horse.