In a manner that involves or is characterized by joint action; in a way that shows collaboration or working together.
From 'coactive' + '-ly' (adverbial suffix). The '-ly' ending transforms adjectives into adverbs describing how something happens, a pattern that became standardized in Middle English.
The '-ly' adverb suffix is so productive that English speakers instantly understand 'coactively' even if they've never seen it before—it's why English is flexible but still comprehensible.
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