In a manner that is abhorrent; in a way that inspires intense hatred, disgust, or strong moral disapproval.
From abhorrent (adjective) plus -ly suffix, making an adverb. Abhorrent comes from Latin abhorrens. The adverbial form allows describing how actions or events inspire revulsion.
Adding -ly to adjectives seems simple, but it creates interesting questions: when we say someone behaves 'abhorrently,' we're judging not just the action but the person's moral character—language packs a lot of judgment into one word.
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