Where does “dörtig” come from?

dörtig (North Frisian) comes from Middle Dutch dertich, from Old Dutch thrītig, from Proto-Germanic þrīz tigiwiz — thirty.

dörtig (North Frisian): thirty

Definitions

Step	Language	Word	Meaning
1	Middle Dutch	dertich	thirty
2	Old Dutch	thrītig	—
3	Proto-Germanic	þrīz tigiwiz	thirty

dörtig — Etymology, Origin & Word History | Etymology Explorer