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to hum (a tune); to sing to oneself; to croon
彼女は散歩しながら懐かしい歌を口ずさんだ。
She hummed a nostalgic song while taking a walk.
朝の支度をしながらつい口ずさんでしまう。
I find myself humming while getting ready in the morning.
子どもが覚えたばかりの童謡を口ずさんでいた。
The child was humming a nursery rhyme just learned.