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To be very willing or happy to do something. Often used to refer to doing some kind of sacrifice
Positive
我心甘情愿把心爱的钢铁侠玩具送给了同学 | Wǒ xīngānqíngyuàn bǎ xīn'ài de gāngtiě xiá wánjù sòng gěi le tóngxué
I was more than happy to give my Iron Man figures to my classmate
[Source]
为了人民的幸福,就是流血牺牲,他也心甘情愿 | Wèile rénmín de xìngfú, jiùshì liúxuè xīshēng, tā yě xīngānqíngyuàn
He is more than willing to put his life down for his people's happiness
[Source]
More than willing/happy to do something
甘心情愿 (gān xīn qíng yuàn)
This is an HSK chengyu
《摭青杂说》(Zhí Qīng Zá Shuō), a work by Sòng Dynasty (宋, 960 - 1279) author 王明清 (Wáng Míng Qīng)
This chengyu is from a line in the work listed in the "Origin" section above