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unintentionally; calmly; inadvertently; innocently
何気なく本を開いた。
I casually opened a book.
何気なく発した一言が相手を傷つけた。
An offhand remark hurt the other person.
彼女は何気なく笑っていたが、実は傷ついていた。
She was laughing as if nothing was wrong, but she was actually hurt.