Where does “theirselves” come from?

theirselves (English) comes from English their, from Old Norse þeirra.

theirselves (English): Alternative form of themselves

Definitions

Step	Language	Word	Meaning
1	English	their	Belonging to, from, of, or relating to, them;...
2	Old Norse	þeirra	—

theirselves — Etymology, Origin & Word History | Etymology Explorer