theiryoudaoicibaDictYouDict[their 词源字典]
their: [12] Like they and them, their was borrowed from Old Norse. Its source was theirra, the genitive plural form of the demonstrative adjective sá. The pronoun form theirs [13] is an English creation.
=> them, they[their etymology, their origin, 英语词源]
their (pron.)youdaoicibaDictYouDict
plural possessive pronoun, c. 1200, from Old Norse þierra "of them," genitive of plural personal and demonstrative pronoun þeir "they" (see they). Replaced Old English hiera. As an adjective from late 14c. Use with singular objects, scorned by grammarians, is attested from c. 1300, and OED quotes this in Fielding, Goldsmith, Sydney Smith, and Thackeray. Theirs (c. 1300) is a double possessive. Alternative form theirn (1836) is attested in Midlands and southern dialect in U.K. and the Ozarks region of the U.S.