海誓山盟纯属玩笑海枯石烂纯属扯淡
[hăi shì shān méng chún shŭ wán xiào hăi kū shí làn chún shŭ chĕ dàn]
'Vows made by sea and mountain are mere jokes; promises until the seas dry up and the rocks crumble are nothing but nonsense.' It expresses skepticism or disappointment towards romantic promises which are usually seen as unattainable exaggerations or empty words.