轻约白首
[qīng yuē bái shŏu]
Combining the meanings, it refers to lightly promising through long hair whitened with age (often metaphorically speaking, 'til death do us part' in traditional Chinese poetry context.) 'White hair/white head' symbolizing longevity, maturing together in love over time rather hastily, gently and gracefully committing throughout lifetime until getting old and having white hair together, which represents fidelity and timeless love.