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to hum (a tune); to sing to oneself; to croon
彼女は楽しそうに好きな歌を口吟んでいる。
She is humming her favorite song happily.
公園を散歩しながら古い歌を口吟んだ。
I hummed an old song while walking in the park.
子供の頃に覚えた歌を懐かしく口吟む。
I nostalgically hum songs I learned as a child.