A form of the possessive case of the pronoun
thou, now superseded in common discourse by
your, the possessive of
you, but maintaining a place in solemn discourse, in poetry, and in the usual language of the Friends, or Quakers. [
1913 Webster]
" In the old style, thine was commonly shortened to thi (thy) when used attributively before words beginning with a consonant; now, thy is used also before vowels. Thine is often used absolutely, the thing possessed being understood."
[1913 Webster]