A flag is a piece of cloth that represents a country, group, or institution. If you're particularly proud of your Irish heritage, you might fly an Irish flag in your front yard.
A real flag enthusiast might fly the American flag, their state flag, their city's flag, and the flag of their parents' birth countries. There are also flags used for signaling on ships at sea and during races and sporting events, and flags that represent armies or advertise products. When flag is used as a verb, it means "to lose energy or enthusiasm." When students start to flag in the afternoon, they tend to take a quick nap — sitting up at their desks.