Source:
https://scmp.com/magazines/hk-magazine/article/2037863/wrap-two-milk
Magazines/ HK Magazine

Wrap Two Milk

吹水 (chui sui), v. Cantonese slang. To chat, bullshit. 

HK Magazine
HK Magazine

Updated: 5:05pm, 19 Oct, 2016

Why you can trust SCMP

“Keep a mistress.”

Siu naai naai (少奶奶)  “little milk,” is a young wife—so a mistress is yi naai—“second wife.”