Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • pro. Used to indicate the one or ones belonging to them: The red house is theirs. If your car doesn't start, take theirs.
  • pro. Usage Problem His or hers: brought his own lunch and expected everybody else to bring theirs. See Usage Notes at he1, they.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • pro. That which belongs to them; the possessive case of they, used without a following noun.

from The Century Dictionary and Cyclopedia

  • See they.

Etymologies

Middle English, from their, their; see their.
(American Heritage® Dictionary of the English Language, Fourth Edition)
their +‎ -'s (Wiktionary)

Examples

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