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unconsciously; involuntarily; instinctively; reflexively; spontaneously; unintentionally; in spite of oneself
びっくりして思わず声を出した。
I was so surprised that I unconsciously let out a voice.
美しい景色に思わず見とれてしまった。
I unconsciously became absorbed in the beautiful scenery.
面白い話を聞いて思わず笑った。
I heard a funny story and laughed unintentionally.