themselves (pron.)youdaoicibaDictYouDict[themselves 词源字典]
mid-15c. in northern dialect, standard from 1540s, alteration of Middle English tham-self, emphatic plural pronoun, also reciprocal pronoun (14c.); see them + self, with self, originally an inflected adjective, treated as a noun with a meaning "person" and pluralized. Displacing Old English heom selfum (dative). Themself returned late 20c. as some writers took to replacing himself with gender-neutral everyone, anyone, etc.[themselves etymology, themselves origin, 英语词源]