To upset means to disturb someone's emotions or disrupt a situation that seemed settled. When your friend gets upset after losing a game, they feel disappointed, frustrated, or sad. When bad news upsets your family's dinner plans, it throws everything into disorder.

The word captures both emotional disturbance and physical disruption. You might accidentally upset a glass of milk, knocking it over and spilling it. You might upset your parents by breaking a promise. A surprising storm can upset travel plans, forcing everyone to change their arrangements.

In sports, an upset is when the team or player expected to lose actually wins. When a small school defeats a champion team, everyone calls it a major upset because it overturns what people predicted would happen.

Notice that upset works as a verb (to upset something), an adjective (feeling upset), and a noun (an upset). When someone has an upset stomach, their digestive system is disturbed and uncomfortable. The word always suggests something being knocked out of its normal, comfortable state, whether that's your emotions, your plans, or a glass of water sitting peacefully on a table.