no obj.
1- hum or sing in a soft, low voice, especially in a sentimental manner（尤指充满感情地）低吟; 轻哼, 哼唱：
she was crooning to the child
她在对孩子轻轻哼着歌
with obj. the female vocalist crooned smoky blues into the microphone.女歌手对着麦克风轻哼着缥缈的布鲁斯歌曲。
1.1
- with direct speech say in a soft, low voice温和低声地说：
'Goodbye, you lovely darling,' she crooned.
“再见, 亲爱的宝贝,"她低声细语地说。
in sing.
- a soft, low voice or tone柔和的嗓音（或音调）：
he sang in a gentle, highly expressive croon.
他用轻柔而极富表情的音调哼唱着。
词源
late 15th cent. (originally Scots and northern English)：from Middle Low German and Middle Dutch krōnen 'groan, lament'. The use of croon in standard English was probably popularized by Robert Burns.