From BiblePortal Wikipedia
King James Dictionary 
Thy, a. contracted from thine, or from some other derivative of thou. It is probable that the pronoun was originally thig, thug or thuk, and the adjective thigen. See Thou.
Thy is the adjective of thou, or a pronominal adjective, signifying of thee, or belonging to thee, like tuus in Latin. It is used in the solemn and grave style.
These are thy works, parent of good.
Thyine wood, a precious wood, mentioned Revelation 18
Webster's Dictionary 
(pron.) Of thee, or belonging to thee; the more common form of thine, possessive case of thou; - used always attributively, and chiefly in the solemn or grave style, and in poetry. Thine is used in the predicate; as, the knife is thine. See Thine.