相濡以沫一诺永久
[xiāng rú yĭ mò yī nuò yŏng jiŭ]
The phrase originates from an ancient Chinese legend where couples of fish living out of water moisten each other using foam. Thus, it signifies mutual care during hard times and lifelong commitment in intimate relationships. In English, you can interpret this as: 'Through thick and thin, keeping promises forever.'