Where does “trái nghĩa” come from?

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

trái nghĩa (Vietnamese): antonymous

Definitions

trái nghĩa traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Vietnamese	trái	left; reverse; morally wrong, unreasonable,...
2	Proto-Vietic	p-laːs	fathom
3	Proto-Mon-Khmer	laas	to spread; to extend

Step	Language	Word	Meaning
1	Vietnamese	nghĩa	a meaning; a sense; morality
2	Chinese	義	—