Where does “ทศ” come from?

ทศ (Thai) comes from Sanskrit दश, from Sanskrit देश, from Proto-Indo-Aryan dayśás, from Proto-Indo-Iranian dayćás, from Proto-Indo-European deyḱ- — to point out.

Ancestry of “ทศ”, step by step

Step	Language	Word	Meaning
1	Sanskrit	दश	ten
2	Sanskrit	देश	point, region, spot, place, part, portion;...
3	Proto-Indo-Aryan	dayśás	point, region, part; country, province
4	Proto-Indo-Iranian	dayćás	part, point; country
5	Proto-Indo-European	deyḱ-	to point out

Every word from Proto-Indo-European deyḱ- →

ทศ — Etymology, Origin & Word History | Etymology Explorer