Verb
(of a ship, airline etc) To be registered in a particular country and display that country's flag as a result.
(idiomatic) To support one's country enthusiastically.
Source: en.wiktionary.orgWe said we'd fly the flag without him and carry on. I didn't give him a kiss because I still hadn't accepted what was happening. I was hoping that some miracle was going to happen. Of course, it didn't. I wish I had kissed him now. Maurice Gibb
I don't want to fly the flag for being unhealthy and overweight, but I don't want to fly the flag for being too thin, either. Martine McCutcheon