从前的海誓山盟
[cóng qián de hăi shì shān méng]
'Past Vows as Firm as Seas and Mountains' refers to grand promises made earlier between partners, indicating commitment and loyalty comparable to immovable natural features like seas and mountains. It often carries connotations about how powerful those vows were once made but have now changed, leading to feelings of regret or loss in retrospection.