Possessive of They.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (pronoun) (pron.) (pronoun)
Poss. of THEY.
- The american dictionary of the english language. By Daniel Lyons. Published 1899.
pron. Of them.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894.
The form of the possessive their used alone without a following noun.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (pronoun) (pron.) (pronoun)

Usage examples:

As I said, we have these men's lives to answer for; and while that fellow lives theirs and our own are equally in danger.
- The League of the Leopard by Harold Bindloss
Dire deeds are theirs to day.
- The Seven Plays in English Verse by Sophocles
Aby Mollett raised his hat, and the two gentlemen touched theirs
- Castle Richmond by Anthony Trollope