O.E. hire, third pers. sing. fem. gen. form of heo "she."
c.1300, hires, from her, a double possessive. Possessive pronouns in Mod. Eng. 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 O.E. and early M.E., they were identical. To
keep speech fluid, speakers began to affix an -n to the end of 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 pronouns in that class usually 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.
c.1154, probably evolved from O.E. seo, sio (acc. sie), fem. of demonstrative pronoun se "the." The O.E. word for "she" was heo, hio, however by 13c. the pronunciation of this had converged by phonetic evolution with he "he," so the fem. demonstrative pronoun probably was used in its place (cf. similar
development in Du. zij, Ger. sie, Gk. he, etc.). The original h- survives in her. A relic of the O.E. pronoun is in Manchester-area dial. oo "she." She-devil "difficult woman" first recorded 1840.