太阳打西边出来我与你私奔
[tài yáng dă xī biān chū lái wŏ yŭ nĭ sī bēn]
Literally meaning 'I'll elope with you if the sun rises in the west', this is an expression emphasizing something that's highly improbable. It suggests extreme conditions under which the person would choose to be together, highlighting their devotion and commitment.