软肋(ruǎnlèi)  soft spot

Usage:

人身体中的软肋是脆弱的，容易遭受破坏，因此“软肋”指人或事情的薄弱之处。
Rib cartilage in the human body is fragile and easily damaged. As such, “软肋” means the weaknesses of someone or something (similar to one’s Achilles’ heel).
Sentences:

1、Zhī qián de nà duàn gǎn qíng shì tā de ruǎn lèi, nǐ jìn liàng bú yào tí.
之前的那段感情是他的软肋，你尽量不要提。
That previous relationship is his weakness; you should try not to mention it.

2、Zhǐyǒu zhuāzhù duìfāng de ruǎnlèi, cái kěnéng zài tánpàn zhōng zhànyǒu zhǔdòng quán. 只
有抓住对方的软肋，才可能在谈判中占有主动权。
only by seizing your opponent’s weakness can you gain the initiative in negotiation.