The word "theirs" represents a key development in the English pronoun system, emerging in the Middle English period to fill a grammatical gap. It is built upon the pronoun "they," which was borrowed from Old Norse "þeir" to replace the Old English plural third-person pronouns, providing a clearer distinction in number. The addition of the genitive suffix "-es," a common marker of possession in Old English (as in "kinges" for "king's"), created the possessive form "theires." Over time, this form contracted and regularized into "theirs," establishing it as the standard absolute possessive pronoun corresponding to "they," used independently without a following noun.

The word "theirs" is a fused, unsegmentable possessive pronoun form in Modern English. It originates from the Middle English "theires," which itself was formed from the plural pronoun "they" (of Old Norse origin) + the genitive suffix "-es" (from Old English).