柠檬真心酸
[níng méng zhēn xīn suān]
Combining the concept of lemon being tart and sour ('真心酸' meaning 'really sour' but also can be used figuratively for a bittersweet feeling), it implies genuine but slightly bitter emotions, perhaps from envy ('lemonade' commonly symbolizes jealousy or bitterness) or unexpressed longing or grievance.