In a chattering manner; while making chattering sounds or speaking rapidly and continuously.
From chatter + -ing (present participle) + -ly (adverb-forming suffix). The word describes how an action is performed in a chattery fashion.
By adding -ingly to the verb, English lets us describe not just the action of chattering but the manner or style of any other action—you could walk 'chatteringly' while gossiping with friends!
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