Used as a modifier before a noun.
(archaic) Of, belonging to, made by, or done by thee.
possessive pronominal adjective
The definition of thy is your, or is the possessive form of thou.
An example of thy is a man making a reference to his friend's wife by saying thy wife.
Origin of thy
- Middle English variant of thin thine from Old English thīn tu- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition