Other forms: gained the upper hand; gaining the upper hand; gains the upper hand
To gain the upper hand is to move into a more powerful position and have an advantage over someone. When one tennis player gains the upper hand, her opponent is suddenly on the defensive.
Etymologists believe that the phrase gain the upper hand comes from an old game in which players take turns grasping a stick, each placing their hand just above the previous player's. The person whose hand grabs the top of the stick is the winner, gaining the upper hand. Figuratively, you gain the upper hand when you're on top — winning a game, outmaneuvering an opponent, or otherwise taking control of the situation.