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.
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Harper, D. (n.d.). Etymology of theirs. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/theirs