theirs

a form of the possessive case of plural they used as a predicate adjective, after a noun or without a noun: Oh, you know Walt and Liz—are you a friend of theirs?

a form of the possessive case of singular they used as a predicate adjective, after a noun or without a noun.

1. (used to refer to a generic or unspecified person previously mentioned, about to be mentioned, or present in the immediate context).

   I have my book—does everyone else have theirs?

2. (used to refer to a specific or known person previously mentioned, about to be mentioned, or present in the immediate context).

   The child couldn’t say whether or not the coat was theirs.

3. (used to refer to a nonbinary or gender-nonconforming person previously mentioned, about to be mentioned, or present in the immediate context).

   I've already gotten my coffee, but Morgan is still waiting to get theirs.

that which belongs to them.