Where does “mát mẻ” come from?

mát mẻ (Vietnamese) comes from Vietnamese mát, from Proto-Vietic t-maːc.

mát mẻ (Vietnamese): cool

Definitions

Step	Language	Word	Meaning
1	Vietnamese	mát	cool; cray-cray
2	Proto-Vietic	t-maːc	—

mát mẻ — Etymology, Origin & Word History | Etymology Explorer