The computer is 'theirs'. [=the computer belongs to them; it is their computer]
He bought his car on the same day his parents bought 'theirs'.
That dog 'of theirs' [=their dog] is always barking.
(..) Cut them down just like they did this boy's people. Aye! They've slaughtered villages. And we've slaughtered theirs. I will follow you anywhere, you know that. - But they killed Grenn. - Yes. - And they killed Pyp. (..)2015 TvShow: Game of Thrones Title: Kill the Boy Season: 5 Episode 5