Where does “mặt trái” come from?

mặt trái (Vietnamese) comes from Vietnamese trái, from Proto-Vietic p-laːs, from Proto-Mon-Khmer laas — to spread; to extend.

mặt trái (Vietnamese): reverse; flip side; backside

Definitions

  1. reverse; flip side; backside

Ancestry of “mặt trái”, step by step

mặt trái traces back through two separate lines of ancestry.

via Vietnamese trái

StepLanguageWordMeaning
1Vietnamesetráileft; reverse; morally wrong, unreasonable,...
2Proto-Vieticp-laːsfathom
3Proto-Mon-Khmerlaasto spread; to extend

via Vietnamese mặt

StepLanguageWordMeaning
1Vietnamesemặtface; aspect; dimension; right
2Middle Vietnamese𩈘face
Every word from Proto-Mon-Khmer laas →Every word from Proto-Vietic p-laːs →Every word from Vietnamese trái →