I. ˈthe](ə)rz, ˈtha(a)], ]əz\ pronoun
Etymology: Middle English theirs, theires, from their + -s, -es -'s
1. : their one : their ones — used without a following noun as a pronoun equivalent in meaning to the adjective their
bought his car the same day his neighbors bought theirs
our customs are not like theirs
— often used after of to single out one or more members of a class belonging to or connected with certain persons not including the one speaking or writing nor the one being addressed
a friend of theirs
some shrubs of theirs
or merely to identify something or someone as belonging to or connected with certain persons not including the one speaking or writing nor the one being addressed without any implication of membership in a more extensive class
that dog of theirs
those flashy clothes of theirs
2. : something belonging to them : what belongs to them
they are determined to get theirs
3. : his or hers : his , hers — used with a singular antecedent that is indefinite or that does not specify gender
I will do my part if everybody else will do theirs
II. adjective
obsolete : their II 1 — used as the first of two possessive adjectives modifying the same noun