Where does “کاستن” come from?

کاستن (Persian) comes from Proto-Iranian kas-, from Proto-Indo-European kʷeḱ- — to see; to look.

Ancestry of “کاستن”, step by step

Step	Language	Word	Meaning
1	Proto-Iranian	kas-	to be small, diminish, lessen
2	Proto-Indo-European	kʷeḱ-	to see; to look