To resist openly or boldly; to refuse to obey; or to be impossible or extremely difficult to do something.
From Old French 'defier' meaning 'to renounce' or 'to challenge,' from 'de-' (reversal) + 'fier' (to trust, from Latin 'fidus'). So literally, to 'de-trust' or formally reject authority.
The word originally meant a formal declaration of war—knights would literally send a 'defiance' before attacking—so when we say someone 'defies' authority, we're using language borrowed from medieval combat etiquette!
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