Theirs

wiktionary

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From Middle English theires, attested since the 1300s. Equivalent to their +‎ -s (compare -'s); formed by analogy to his. Displaced theirn (from Middle English theiren, formed by analogy to mine, thine) in standard speech. [1]

etymonline

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theirs (pron.)

possessive pronoun, "their own," early 14c., from their + possessive -s, on analogy of his, etc. In form, a double possessive.