一yī往wǎng情qíng深shēn

Literal: going wholly toward it, deep feeling
Figurative: To feel a deep, wholehearted devotion to someone or something that one cannot shake off; not love at first sight, but a profound and steadfast attachment.

Where does 一往情深 come from?

It comes from the 《世说新语》, the collection of anecdotes compiled by Liu Yiqing in the fifth century. It tells how Huan Yi, known as Ziye, could not hold back whenever he heard a beautiful song and would cry out, "Alas, what can one do!" On hearing of this, the statesman Xie An remarked that Ziye could be said to feel things to the very bottom once they touched him. The idiom grew out of that remark. Today it is used mostly for faithful, lasting love for a person, sometimes unrequited, and also for attachment to a place, an art or a calling.

Source (出处): 《世说新语·任诞》

How is 一往情深 used? Two examples

两liǎng人rén不bù是shì一yī见jiàn钟zhōng情qíng，但dàn结jié婚hūn三sān十shí年nián，他tā对duì她tā始shǐ终zhōng一yī往wǎng情qíng深shēn。
It wasn't love at first sight, but through thirty years of marriage he has stayed deeply devoted to her.
老lǎo先xiān生sheng对duì昆kūn曲qǔ一yī往wǎng情qíng深shēn，退tuì休xiū后hòu每měi周zhōu都dōu去qù戏xì园yuán子zi听tīng戏xì。
The old gentleman is utterly devoted to Kunqu opera; since retiring he goes to the theater every week to hear it.

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