Old English eower, possessive pronominal adjective, genitive of ge "ye" (see ye), from Proto-Germanic base of you. Cognate with Old Saxon iuwar, Old Frisian iuwer, Old Norse yðvarr, Old High German iuwer, German euer, Gothic izwar "your." Used in titles of honor by mid-14c.
Old English hire "her," third person singular feminine dative pronoun, which replaced accusative hie beginning in 10c. See he. Cognate with Old Frisian hiri, Middle Dutch hore, Dutch haar, Old High German iru, German ihr.
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Harper, D. (n.d.). Etymology of yourn. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/yourn