theirselves

From their +‎ -selves, patterned on ourselves and similar terms.

theirselves (singular himself or hisself or herself)

1. (nonstandard) Alternative form of themselves
   • 1776, Jeremy Bentham, “A Short Review of the Declaration”, in John Lind, An Anſwer to the Declaration of the American Congress‎^[1], London: Thomas Cadell, page 121:

     Or would they have it believed, that there is in their ſelves ſome ſuperior ſanctity, ſome peculiar privilege, by which theſe things are lawful to them, which are unlawful to all the world beſides?

   • 1961, Bill Peet, One Hundred and One Dalmatians, Walt Disney Productions:

     Horace: "Look, Jasper! Do you supposed they disguised theirselves?"

• shirt sleeve, shirt-sleeve, shirtsleeve, silver sheet, silver-sheet