Defy descends from Old French desfier, from Vulgar Latin *disfīdāre — literally 'to un-trust' or 'to renounce faith.' In feudal society, defiance was a solemn legal act: a vassal breaking his oath of fealty before taking up arms. Its semantic arc — from renouncing an oath to bold resistance — mirrors the decline of feudal obligation and the rise of individual autonomy.
To openly resist or refuse to obey; to challenge someone to do something believed impossible; to resist boldly.
From Old French 'desfier' (to challenge, to renounce one's faith or allegiance, to declare war), from Vulgar Latin *disfīdāre (to un-pledge, to withdraw trust), a compound of 'dis-' (away, reversal, negation) + 'fīdāre' (to trust, to entrust), from 'fīdēs' (faith, trust, good faith). The PIE root is *bʰeydʰ- (to trust, to urge, to persuade, to command). The literal meaning of 'defy' is 'to un-faith' — to withdraw the pledged trust that binds lord
In medieval feudal law, to 'defy' your lord was a formal legal act called diffidatio — a vassal publicly renouncing his oath of fealty before taking up arms. Without this formal renunciation, armed resistance was mere rebellion; with it, warfare could be legitimate. The word literally meant 'to un-faith