From Longman Dictionary of Contemporary Englishtheirstheirs /ðeəz $ ðerz/ ●●●S3 pronoun [possessive form of ‘they’] 🔊 🔊 1PERSON/PEOPLEused to refer to something that belongs to or is connected with people that have already been mentioned 🔊 When our washing machine broke, our neighbours let us use theirs. 🔊 Our house is number 25, and theirs is just opposite. 🔊 We compared scores and found that theirs were higher than ours.of theirs 🔊 They shared the prize money with a friend of theirs.2PERSON/PEOPLEused when talking about someone who may be male or female, to avoid saying ‘his or hers’ 🔊 Everyone wants what is theirs by right. →his(2)
Examples from the Corpus
friend of theirs• Well, apparently the boyfriend is a friend of theirs.• Anna was talking to an old friend of theirs, Irena Ignatiev.• Finally some friends of theirs who had a severely handicapped child told Mike this was illegal: that they had some rights.