her objective case

"the female person referred to," third person nominative fem. pronoun, used as a substitute for the name of a female or anything regarded as female, mid-12c., probably evolving from Old English seo, sio (accusative sie), fem. of the demonstrative pronoun (masc. 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 (compare 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 human being, a woman," she is attested from early 14c. Also used to signify "female" with the names of other creatures (late 14c.; she-wolf, etc.). The attempted gender-neutral pronoun form s/he is attested by 1977.

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 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.