Where does “śilā” come from?

śilā (Old Javanese) comes from Sanskrit शील, from Sanskrit शी, from Proto-Indo-European ḱey- — to be lying down; to settle.

Ancestry of “śilā”, step by step

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

śilā — Etymology, Origin & Word History | Etymology Explorer