Origin of theirself
Examples from the Web for theirselves
Those who think so little of theirselves seldom find happiness though—do they, sir?Mattie:--A Stray (Vol 3 of 3)|Frederick William Robinson
And I've heard tell that they have regular high jinks up there all by theirselves, and vittles!The Old Tobacco Shop|William Bowen
As for any attractive influences of supper, those children never come in of theirselves, and always had to be fetched.When Ghost Meets Ghost|William Frend De Morgan
Wrap it in skirts an' young fellers make plumb fools o' theirselves.Bring Me His Ears|Clarence E. Mulford
I always was fond o' childern, but if 't 'd bin older ones able to look after theirselves I shouldn't ha' took so much notice.Memoirs of a Surrey Labourer|George Sturt (AKA George Bourne)
c.1300, variant of themself, with self, originally an inflected adjective, treated as a noun with a meaning "person." Related: Theirselves.