'Defiance' is Latin for 'un-faithing' — breaking feudal loyalty, from 'fides' (faith).
From Old French 'defiance' (challenge to combat, formal declaration of hostility, renunciation of allegiance), from 'defier' (to defy, to challenge, to renounce allegiance to a lord), from Vulgar Latin *disfīdāre (to renounce one's sworn faith, to dissolve a fealty bond), compounded from Latin 'dis-' (apart, away, negation, undoing) and 'fīdēs' (faith, trust, loyalty, sworn allegiance). The PIE root underlying 'fīdēs' is *bʰeydʰ- (to trust, to confide, to persuade, to compel through trust). The feudal context is crucial to understanding the word's original meaning: in medieval law, 'defiance' was the formal renunciation of fealty — a vassal's solemn declaration that he no longer
'Defiance' is literally 'un-faithing' — breaking one's oath of loyalty. From the same root as 'faith,' 'fidelity,' 'confidence,' and 'federal' (all from Latin 'fīdēs,' trust). To defy someone originally meant to renounce your allegiance to them — to declare the feudal bond broken. Defiance was divorce from a lord.