Thy

• POS: Definite Article

CIDE DICTIONARY

Thy, pron. [OE. thi, shortened from thin. See Thine, Thou.].

 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.  [1913 Webster]
"Our father which art in heaven, Hallowed be thy name. Thy kingdom come. Thy will be done."  [1913 Webster]
"These are thy glorious works, Parent of good."  [1913 Webster]

OXFORD DICTIONARY

Thy, poss.pron. (attrib.) (also thine before a vowel) of or belonging to thee: now replaced by your except in some formal, liturgical, dialect, and poetic uses.

Etymology

ME thi, reduced f. thin THINE