Where does “ម៌្មាស៑” come from?

ម៌្មាស៑ (Old Khmer) comes from Proto-Mon-Khmer rmaas — rhinoceros.

Ancestry of “ម៌្មាស៑”, step by step

Step	Language	Word	Meaning
1	Proto-Mon-Khmer	rmaas	rhinoceros

ម៌្មាស៑ — Etymology, Origin & Word History | Etymology Explorer