If you act defiantly, you’re a rebel. On "Everyone Wear Red" Day, you show up defiantly dressed in head-to-toe blue, or white, or, well, anything but red.
Defiantly is an adverb that is linked to the noun defiance which is defined as "bold disobedience." It is one thing to behave badly and hope to get away with it. It is quite another to want to be seen behaving badly — that's action done defiantly: it defies, or openly goes against an order or rule. People who act defiantly might have a good reason — or they might just want attention.