Where does “threty” come from?

threty (Scots) comes from Middle English thretty, from Old English þritiġ, from Proto-Germanic þrīz tigiwiz — thirty.

threty (Scots): thirty

Definitions

Step	Language	Word	Meaning
1	Middle English	thretty	—
2	Old English	þritiġ	thirty
3	Proto-Germanic	þrīz tigiwiz	thirty

threty — Etymology, Origin & Word History | Etymology Explorer