THEIR, a pronom.
1. Their has the sense of a pronominal adjective, denoting of them, or the possession of two or more as their voices their garments their houses their land their country.
2. Theirs is used as a substitute for the adjective and the noun to which it refers, and in this case, it may be the nominative to a verb. "Our land is the most extensive, but theirs is the best cultivated." Here theirs stands as the representative of their land, and is the nominative to is.
Nothing but the name of zeal appears
'Twixt our best actions and the worst of theirs.
In this use, theirs is not in the possessive case, for then there would be a double possessive.
The King James Bible has stood its ground for nearly 400 years. However, during that time the English language has changed, and with it the meanings of some words it used. Here are more than 6,500 words whose definitions have changed since 1611.Wikipedia
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