Where does “แม่น้ำ” come from?

แม่น้ำ (Thai) comes from Thai น้ำ, from Proto-Tai *C̬.namꟲ.

แม่น้ำ (Thai): river

Definitions

Step	Language	Word	Meaning
1	Thai	น้ำ	water; any liquid or fluid; body of water, as...
2	Proto-Tai	*C̬.namꟲ	—

แม่น้ำ — Etymology, Origin & Word History | Etymology Explorer