A flag, as I'm sure you know, is a piece of cloth used as a symbol of a country or organisation. Such flags can also be used to send messages or signals. You can flag up something for attention, or flag down a taxi.
Flat stones used for paving are sometimes known as flags. To flag is to become tired or less enthusiastic. Flag is also a type of iris.
Is there anything you can think of that I should have flagged, but didn't? Ah yes, another free to enter writing contest.