"Thy" is an English word that means "your" in the second person singular. English used to have a distinction between singular and plural in the second person, such that we had the following: Singular: thou, thee, thy. Plural: ye, you, your.
The determiner thy is the possessive adjective form of “thou.” It's a second-person possessive adjective. Where “thou” is a subject that means “you,” thy means “your.” Thy always comes before a noun. For example, “thy body” means “your body.”
You shouldn't, because those are archaic words, but if you need to use them for some reason, you use thy and thine for the other person, and my and mine for yourself.
Thy is an old-fashioned, poetic, or religious word for `your' when you are talking to one person. Honor thy father and thy mother.
Possessive thy and the subject/object forms thou/thee are old forms which are no longer used in Standard English, and are dying out even in the dialects where it has survived.
Reasons commonly maintained by modern linguists as to the decline of thou in the 17th century include the increasing identification of you with "polite society" and the uncertainty of using thou for inferiors versus you for superiors (with you being the safer default) amidst the rise of a new middle class.
"Thy" is an English word that means "your" in the second person singular. English used to have a distinction between singular and plural in the second person, such that we had the following: Singular: thou, thee, thy.
According to A History of the English Language, “In the thirteenth century, the singular forms (thou, thy, and thee) were used in addressing children or persons of inferior rank, while the plural forms (ye, your, and you) began to be used as a mark of respect in addressing a superior (235).
: of or relating to thee or thyself especially as possessor or agent or as object of an action. used especially in ecclesiastical or literary language and sometimes by Friends especially among themselves.
Thou, thee, thy, thine and ye are archaic personal pronouns which are generally articulated in the form of subject and object. Thou is a singular informal subjective case. Thou means you, however, it is analogous to the use of he and I in modern English.
Thy is used instead of the word your: thy house, thy dog, thy book. Thine is used before words beginning with a vowel: thine apple. Ye is used instead of saying all of you: Ye all shalt come.
There is no indication in the text of any early English translations that the use of thou/thee/thy/thine signifies "informal you". That notion is a theological and interpretative one, and frankly better asked about elsewhere (SE: Christianity or SE: Biblical Hermeneutics).
During the 1600's. These familiar second person singular pronouns remained in some rural British dialects for a long time after that.
Those terms stopped being used in standard English in the 17th century.
“Thou” means “you” when “you” is the subject of the sentence: “Thou art most beauteous” “Thy” means “your” when used before a word beginning with a consonant: “Hallowed be thy name” “thy kingdom come” “Thine” means “your” when used before a word beginning with a vowel: “Drink to me only with thine eyes.”
How to use Thy in a sentence. May each evening see that all thy wishes have been performed. Both worlds are illumined by thy rays. Let not to get a living be thy trade, but thy sport.
While "thee" and "thou" (and the corresponding verb forms such as "shalt") sound formal to us because they're associated with the Bible, they were originally the informal or intimate versions of of the second person pronoun, used either with kin and close friends or from superior to inferior.
We still see thou is some forms of modern use, such as in discussions of the “I and Thou” concept of Martin Buber's philosophy, or in colloquial phrases such as “holier-than-thou.” For the most part, at least in normal linguistic use, thou has been largely supplanted in modern times by you, although it does exist still ...
I originates from Old English (OE) ic, which had in turn originated from the continuation of Proto-Germanic *ik, and ek; The asterisk denotes an unattested form, but ek was attested in the Elder Futhark inscriptions (in some cases notably showing the variant eka; see also ek erilaz).
Thy and thine are archaic forms corresponding to your and yours respectively. Use thy where you would use your (but see note at end of answer) and thine where you would use yours.
From Middle English þi, apocopated variant of þin, from Old English þīn, from Proto-West Germanic *þīn, from Proto-Germanic *þīnaz, from Proto-Indo-European *téynos (“thy; thine”), from Proto-Indo-European *túh₂ (“thou”). See thou.
Etymology 1
From Middle English mi, my, apocopated form of min, myn, from Old English mīn (“my, mine”), from Proto-West Germanic *mīn, from Proto-Germanic *mīnaz (“my, mine”, pron.)
pronoun [v PRON, prep PRON] Thee is an old-fashioned, poetic, or religious word for `you' when you are talking to only one person. It is used as the object of a verb or preposition.