Relating to opposition or contradiction; contrary in nature or effect.
From anti- (against) + -nial (from Latin -ialis, relating to). Formed in English to describe something that stands against or opposes a particular quality or condition.
This word is rarely used in modern English because we have so many other ways to express opposition—like 'contrary' or 'opposing'—but it shows how English speakers in the 17th-18th centuries were obsessed with creating 'anti-' words to name everything that went against something else.
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