海誓山盟[hǎi shì shān méng]
exchange solemn vows and pledges; lovers pledge of eternal loyalty until the seas dry up and rocks decay; make a solemn pledge [oath] of (eternal) love; Oaths high as the mountains and vows deep as the ocean were exchanged.; plight one's troth; swear by th
例：虽未成双, 却也海誓山盟, 私传表记, 已有无限的风情了。
Although they had not made love, they had secretly exchanged solemn vows and pledges， bearing their hearts to each other.
海誓山盟, 各无他志。
They vowed solemnly never to love anyone else.