In a chatty or talkative manner; while chatting or in a friendly conversational style.
From chatty (adjective) + -ly (adverb-forming suffix). The 'y' changes to 'i' before adding the suffix, just as it does in comparative/superlative forms.
The adverb 'chattily' is interesting because it describes not just talking but the social manner of talking—you can eat chattily (eating while chatting), dress chattily (dressing in a casual, friendly style), or do anything else chattily!
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