To upend something means to turn it completely upside down or on its end. If you upend a bucket, you flip it over so the bottom points up and everything inside spills out. When you upend a glass, the opening that was at the top now faces the floor.

The word also describes overturning ideas, plans, or situations in dramatic ways. A surprise snowstorm might upend your plans for an outdoor birthday party, forcing everyone to celebrate inside instead. A new scientific discovery can upend old theories that scientists had believed for years. When someone says their life was upended by moving to a new city, they mean everything familiar got turned on its head: a new school, new friends, new routines.

Upend suggests more than small changes. It's not adjusting your schedule slightly because dinner runs late. It's finding out that the entire day needs rearranging because something unexpected happened. The word captures that feeling when something stable suddenly gets flipped around, like when you carefully build a tower of blocks and your little brother upends the whole thing with one sweep of his arm.