Origin of theirself
Examples from the Web for theirself
“We live in a society where there are girls who are in love with theirself and flaunt it but the majority are insecure,” says one.Cara Delevingne Posts Protest Picture Of Her Boob (With Taped-Out Nipple)|Tom Sykes|July 14, 2014|DAILY BEAST
Sometimes theirself is substituted for theirselves, as in "they all seen it theirself."The American Language|Henry L. Mencken
And when they were come to theirself again they went to the good man and prayed him that he would say them truth.Le Morte D'Arthur, Volume II (of II)|Thomas Malory
c.1300, variant of themself, with self, originally an inflected adjective, treated as a noun with a meaning "person." Related: Theirselves.