Why is the ninth month called September?
c.1300, hires, from her; 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 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.
Old English hire, third person singular feminine dative pronoun, which beginning in 10c. replaced accusative hie (see he). Cognate with Old Frisian hiri, Middle Dutch hore, Dutch haar, Old High German iru, German ihr.
Old English hire, third person singular feminine genitive form of heo "she" (see she).
mid-12c., probably evolving from Old English seo, sio (accusative sie), fem. of demonstrative pronoun se "the," from PIE root *so- "this, that" (see the). The Old English word for "she" was heo, hio, however by 13c. the pronunciation of this had converged by phonetic evolution with he "he," which apparently led to the fem. demonstrative pronoun being used in place of the pronoun (cf. similar development in Dutch zij, German sie, Greek he, etc.). The original h- survives in her. A relic of the Old English pronoun is in Manchester-area dialectal oo "she." As a noun meaning "a female," she is attested from 1530s.