Pronunciation: (&thslash;ârz),[key] —pron. 1. a form of the possessive case of they used as a predicate adjective, after a noun or without a noun: Are you a friend of theirs? It is theirs. 2. (used after an indefinite singular antecedent in place of the definite masculine form his or the definite feminine form hers): I have my book; does everyone else have theirs? 3. that which belongs to them: Theirs is the largest house on the block.