The possessive case of the personal pronoun
they;
as, their houses; their country. [
1913 Webster]
" The possessive takes the form theirs (/) when the noun to which it refers is not expressed, but implied or understood; as, our land is richest, but theirs is best cultivated."
[1913 Webster]
"Nothing but the name of zeal appears
'Twixt our best actions and the worst of theirs."
[1913 Webster]