Their (?), pron. & a. [OE. thair, fr. Icel. �xed;eirra, �xed;eira, of them, but properly gen. pl. of the definite article; akin to AS. [eth]ara, [eth]�xd6;ra, gen. pl. of the definite article, or fr. AS. [eth]�xd6;ra, influenced by the Scandinavian use. See That.]
The possessive case of the personal pronoun they; as, their houses; their country.
⇒ The possessive takes the form theirs () when the noun to which it refers is not expressed, but implied or understood; as, our land is richest, but theirs is best cultivated.
Nothing but the name of zeal appears
'Twixt our best actions and the worst of theirs.
Denham.
© Webster 1913.