Where does “ជាស្រេច” come from?

ជាស្រេច (Khmer) comes from Khmer ជា, from Thai ช้ำ.

ជាស្រេច (Khmer): already

Definitions

Step	Language	Word	Meaning
1	Khmer	ជា	that; "marks an adverbial phrase", in a ......
2	Thai	ช้ำ	—

ជាស្រេច — Etymology, Origin & Word History | Etymology Explorer