In a completely or utterly extreme manner; used to intensify the description of something as thoroughly bad or wrong.
From 'arrant' + '-ly' (adverb-forming suffix). Arrant itself has a complex history from 'errant' with a shifted meaning.
You'll find 'arrantly' in classic literature where authors want to emphasize moral condemnation—like when someone is described as 'arrantly false' or 'arrantly dishonest,' it carries a rhetorical weight that simple 'false' or 'dishonest' cannot match.
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