thine (pron.)

Old English þin, possessive pronoun (originally genitive of þu "thou"), from Proto-Germanic *thinaz (source also of Old Frisian, Old Saxon thin, Middle Dutch dijn, Old High German din, German dein, Old Norse þin), from PIE *t(w)eino-, suffixed form of second person singular pronominal base *tu-. A brief history of the second person pronoun in English can be found here; see also thou.