Where does “rahsia” come from?

rahsia (Classical Malay) comes from Sanskrit रहस्य, from Proto-Indo-Iranian raȷ́ʰ- — to abandon; secret, mystery; to abandon.

Ancestry of “rahsia”, step by step

Step	Language	Word	Meaning
1	Sanskrit	रहस्य	secret, private, clandestine, concealed,...
2	Proto-Indo-Iranian	raȷ́ʰ-	to abandon; secret, mystery; to abandon

rahsia — Etymology, Origin & Word History | Etymology Explorer