Yong Sui

It means black face. No, not the colour black. It refers to the unpleasant ugly face people have when they’re generally in a bad mood, unhappy, unsatisfied, or unimpressed. Black face is most notably and commonly exhibited by restaurant waiters and retail workers.

Jason: Hey Lisa, why the yong sui today?
Lisa: I’m having my menses!!

Or

Retail employee: *black face*
Customer: Having a bad day?
Retail employee: Mmm.