听说心酸时吃柠檬不会酸
[tīng shuō xīn suān shí chī níng méng bù huì suān]
This nickname plays with the concept that when one is already feeling bitter from heartache, eating something sour like a lemon would not cause any extra bitterness. It implies a person who is going through sadness and has developed a certain immunity to further emotional pain.