Where does “sila” come from?

sila (Malay) comes from Classical Malay sila, from Sanskrit शील, from Sanskrit शी, from Proto-Indo-European ḱey- — to be lying down; to settle.

Ancestry of “sila”, step by step

Step	Language	Word	Meaning
1	Classical Malay	sila	—
2	Sanskrit	शील	conduct, disposition, nature, tendency; custom,...
3	Sanskrit	शी	to lie down
4	Proto-Indo-European	ḱey-	to be lying down; to settle