In a manner that counteracts or opposes something; in a way that works against or neutralizes an effect.
From 'counteracting' (present participle of counteract) plus the adverb suffix '-ly', creating an adverb that describes how an action opposes or works against something.
This is a rare adverb that academics use—'the medicine acts counteractingly on the fever' means it works against the fever, though most writers would just say 'against' instead!
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