Origin and history of thine

the now-little-used 2nd nominative singular personal pronoun, Old English þu, from Proto-Germanic *thu (source also of Old Frisian thu, Middle Dutch and Middle Low German du, Old High German and German du, Old Norse þu, Gothic þu), from PIE *tu-, second person singular pronoun (source also of Latin tu, Irish tu, Welsh ti, Greek su, Lithuanian tu, Old Church Slavonic ty, Sanskrit twa-m).

It was superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. early Quakers). The plural at first was used in addressing superior individuals, later also (to err on the side of propriety) strangers, and ultimately all equals. By c. 1450 the use of thou to address inferiors gave it a tinge of insult unless addressed by parents to children, or intimates to one another. Hence the verb meaning "to use 'thou' to a person" (mid-15c.).

Avaunt, caitiff, dost thou thou me! I am come of good kin, I tell thee!

["Hickscorner," c. 1530]

A brief history of the second person pronoun in English can be found here. Related: Thou-self "you, yourself." Archaic language preserves the enclitic use of it, attached to a preceding auxiliary, artou? hastou?

"of or pertaining to thee," possessive pronoun of the second person singular, late 12c., reduced form of þin (see thine), until 15c. used only before consonants except -h-. Compare mine/my, an/a. It is the possessive of thou, in modern use only in solemn and grave style.

dialectal possessive pronoun from your, attested from late 14c., with -n as in mine, thine, etc.

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 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 class added an -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.