the word 'thine' originates from the old english 'þīn', the genitive form of the second person singular pronoun 'þū' (thou). it is part of the old english system of grammatical case, where 'þīn' functioned as both a possessive pronoun and a possessive adjective. the modern form 'thine' developed through the middle english period, with the final '-n' retained specifically for use before vowel sounds, mirroring the pattern of 'mine' versus 'my'. this archaic pronoun belongs to the germanic family of languages, ultimately deriving from the proto-indo-european root tū- (you).

the word is a simplex form. it is a variant of the pronoun 'thy', with the final '-n' likely added by analogy with 'mine'.