Something that belongs to or is associated with people or things previously mentioned (a possessive pronoun).
From Old English 'thǽra' (of them), combining 'they' with a possessive suffix. It evolved from Germanic roots and has been used to show group ownership since before the 12th century.
English is unusual among major languages because it uses the same word 'theirs' for both singular and plural 'they'—most other languages distinguish between 'their (one person)' and 'their (multiple people)'!
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