Origin and history of themself

emphatic pronoun, plural of himself, herself, itself; mid-15c. in northern dialect, standard from 1540s; an alteration of Middle English tham-self, theim-self, emphatic plural pronoun, also reciprocal pronoun (14c.); see them + self. The self, originally an inflected adjective, came to be treated as a noun with a meaning "person" and pluralized.

It displaced Old English heom selfum (dative). Themself returned late 20c. as some writers took to replacing himself with gender-neutral everyone, anyone, etc.