Theirs
来自Big Physics
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wiktionary
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
theirs (pron.)
possessive pronoun, "their own," early 14c., from their + possessive -s, on analogy of his, etc. In form, a double possessive.
