www.alphadictionary.com

Seems pretty apparent, since it's such a familiar word, but the Spanish translation is desafiante, which I could see is related to the word for "to have faith in, to trust," suggesting that a defiant person has no faith in an authority figure's willingness to exercise their authority. (This came to mind when I saw DESAFIANTE as the headline on a Spanish-language newspaper, over a photo of Congressman Charlie Rangel.)

As you say, a pretty standard word, but one with a colorful history. It has flipped in meaning. Originally it meant to be disloyal, to go back on one's faith. Now it means to challenge, to have faith in oneself, so to speak.