In a contrary or opposite manner; directly against or facing opposition.
From Latin ex- (out) and adverso (toward adversity, from adversus meaning 'turned against'). This phrase literally means 'in opposition' and was used in medieval legal and philosophical texts.
Medieval lawyers loved this phrase because it sounded impressively Latin when they wanted to argue 'on the contrary'—it's essentially 'ad hoc' meets 'adversarial' in a single word.
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