theirself

[th air-self]

pronoun Nonstandard.

Also their·selves

[th air-selvz] /ˌðɛərˈsɛlvz/.

Origin of theirself

1250–1300; Middle English; formed on analogy of myself

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

An' I didn't tell anyone else about it; an' if they found it out for theirselves, I was angry.
Murder Point
Coningsby Dawson
"I canna see what they want drownin' theirselves for," said Morel.
Sons and Lovers
David Herbert Lawrence
Just like an awful great pair of trousers walking by theirselves!
Trapped by Malays
George Manville Fenn
The fact that many persons actually say hisself and theirselves.
A Handbook of the English Language
Robert Gordon Latham
But that is the way with all of them men, thinking so much of theirselves, and that it's but ask and have.
Ruth
Elizabeth Cleghorn Gaskell

c.1300, variant of themself, with self, originally an inflected adjective, treated as a noun with a meaning "person." Related: Theirselves.