c. 1300, hires, from her; thus a double possessive. Possessive pronouns in Modern English consist of the predicative (mine, thine, his, ours, yours, theirs) that come after the subject, and the attributive (my, thy, his, her, our, your, their) that come before it. In Old English and early Middle English, they were identical. To keep speech fluid, speakers began to affix an -n to the end of predicative my and thy before words that began with vowels. This began late 13c. in the north of England, and by 1500 was standard.
Then the predicative and attributive pronouns split, and the remaining pronouns in that class took up -s, the regular affix of possession. But the non-standard speech of the Midlands and south of England extended -n throughout (hisn, hern, yourn), a habit attested from 14c. and more regular than the standard speech, which mixes -s and -n.