Where does “លាសិនហើយ” come from?

លាសិនហើយ (Khmer) comes from Khmer លា, from Proto-Mon-Khmer *laʔ, *laʔ-s.

Ancestry of “លាសិនហើយ”, step by step

Step	Language	Word	Meaning
1	Khmer	លា	donkey, ass; to say goodbye, leave; to quit,...
2	Proto-Mon-Khmer	laʔ, laʔ-s	—