情书不朽磨成沙漏
[qíng shū bù xiŭ mó chéng shā lòu]
The meaning can be understood as 'love letters become immortal, like an hourglass'. It metaphorically suggests that although love letters are physically perishable over time (like sand running through an hourglass), the sentiment they carry can last forever, implying timeless love or undying passion.