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