"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 Nowadays, we just have "you" and "your" in place of those six distinct words (which is why in nonstandard English, we have things like "y'all" or "youse" to distinguish 2nd-person plural from 2nd-person singular).
english.stackexchange.com/users/449/Kosmonaut
I believe it's still used in parts of Yorkshire.
I me
my mine Thou thee thy thine
(You you your yours)
He him
his his She her her hers
We us our ours
Ye
you your yours (You you your yours)
They them their theirs
english.stackexchange.com/users/1696/TRiG
Basically, it means "your".
If you want more detailed definition, check on the Merriam-Webster site : archaic : 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
english.stackexchange.com/users/570/codingbear
Archaic.
From dictionary.com .
thy –pronoun the possessive case of thou (used as an attributive adjective before a noun beginning with a consonant sound): thy table.
thou –pronoun Archaic (except in some elevated or ecclesiastical prose) .
the personal pronoun of the second person singular in the nominative case (used to denote the person or thing addressed): Thou shalt not kill.
english.stackexchange.com/users/165/grokus
‘Thy’ is an old word meaning ‘ your ’ that was used for talking or writing to one person.
english.stackexchange.com/users/127/Mehper C. Palavuzlar
Not really answering the question, but adding some cultural context.
One
(maybe the only) place
most people meet these words nowadays is in old hymns:
Thine be the glory Great
is thy faithfulness
How great thou art
Just a closer walk with thee
Oh come all ye faithful
english.stackexchange.com/users/529/Benjol
The OED has a lot to say about thou , pron and n¹ .
Here is just a very small bit of that: History of use of forms
In Old English, the inherited distinctive 2nd singular forms (including the other members of that paradigm, i.e. thee pron .
, thine adj .
and pron ., thy adj .)
were in regular use independently of register or the status of the addressee.
In Middle English they were gradually superseded by the plural ye, you, your, yours , which occurred with singular reference originally for reasons of showing respect, deference, or formality, but gradually became the usual forms in the standard language.
On the details of this process see discussion at you pron .
, adj ., and n .
Although still occurring in religious contexts (in prayers or hymns addressing God) and in archaic language, later use of the th- forms in ordinary speech has been largely restricted to regional English (now chiefly in the north of England).
The forms were formerly also employed by Quakers in addressing a single person as a mark of equality, a feature which had largely fallen out of use by the 20th century.
While use of thee pron .
as subject form occurs early and is frequent in regional use (see thee pron .
II.), use of thou pron .
as object form is rare; Eng.
Dial.
Dict.
records use (as a stressed form) in northern regional English, Surv.
Eng.
Dial.
records use in Wiltshire, and Sc.
National Dict.
records use in Orkney.
Is also says: Form history
As with other pronouns, the form history of this word is affected by its frequent position in low stress and the development of unstressed beside stressed variants; compare the early development of proclitic and enclitic forms (see Forms 2a, 2b).
It is assumed that in Old English þ—ᵗ the length of the vowel depended on the degree of stress in the sentence (compare R. M. Hogg Gram.
Old Eng.
(1992) I. §5.198).
The modern standard diphthongal pronunciation ( /ðaʊ/ ) reflects a stressed form; compare Middle English þuu , þou (see Forms 1α).
Regional forms, on the other hand, often reflect a reduced vowel (compare e.g. early modern English and regional tha ); such reduction is seen earliest in the Old English enclitic forms -to , -ðo (see Forms 2a).
The modern standard voiced pronunciation of the initial consonant reflects late Middle English lenition of the originally voiceless dental fricative in unstressed words (also found in other pronouns, e.g. this pron .
and adj ., that pron.¹ ).
Original voicelessness is reflected by forms with initial t- (see Forms 2β), which occur even when preceded by voiced consonants.
Such forms originally arose by assimilation to preceding dental stops ( d , t ) or dissimilation from preceding sibilant fricative ( s ), and accordingly occur especially when the forms are enclitic, where often the verbal endings -s , -t , -st preceded them (see Forms 2a, and compare discussion at T n.⁷ ).
In Middle English, such spellings are particularly frequent after d or t in texts of the 13th cent.
and grow less frequent with standardization.
(Insular Scots (now chiefly Shetland) du, doo , on the other hand, shows the influence of Norn.)
Probably the the most important part of all that is that in modern times it is restricted to regional use in the north of England, and perhaps in Orkney.
It still gets used for other things, though.
english.stackexchange.com/users/2085/tchrist
This is a very basic paradigm in English, with roots, and suffixes, and irregularities.
Paradigms are to make grammar easier to remember.
This is not rocket science.
If you speak English, you already know the paradigm for I, me, my, mine.
It's the same paradigm (except for the nominative) with thou, thee, thy, thine .
Note that me, my, and mine rhyme with thee, thy, and thine .
That's your mnemonic for usage, Use thou, thee, thy, and thine exactly the same ways as you would use I, me, my, and mine , respectively.
I and thou are both subject pronouns, and the others all rhyme.
english.stackexchange.com/users/15299/John Lawler
I read a sentence containing the word  thy , but I cannot find the meaning of that word.    Is it older English, or is it still used in contemporary English today?
What does "thy" mean?
english.stackexchange.com
<meaning><pronouns><dialects><early-modern-english><thou-thee-thy>
english.stackexchange.com/questions/1083
english.stackexchange.com/users/252/kiamlaluno
"Thy"is an English word that means "your" in the the second person singular. However, in modern times, it is largely restricted to regional use in the north of England or Orkney. "Thy" is an English word that means "your" in the second person singular but is only used nowadays in northern England and parts of Scotland.
"Thy"is an English word that means "your" in the the second person singular. However, in modern times, it is largely restricted to regional use in the north of England or Orkney. ‘Thy’ is an old word meaning ‘ your ’ that was used in the second person singular.  It is the possessive case of thou  (used as an attributive adjective before a noun beginning with a consonant sound): thy table.  
‘Thy’ is an old word meaning ‘ your ’ that was used for talking or writing to  one  person. It is still used in parts of Yorkshire. Thy is an old English word meaning "your" and was used when talking to one person. It may still be in use in parts of Yorkshire.
"Thy" is an English word that means "your" in the second person singular.   It is used nowadays only in the north of England and parts of Scotland. Thou, thee, thy, thine equate to the first person forms I, me, my, mine.
‘Thy’ is an old word meaning ‘ your ’ that was used in the second person singular.   It is the possessive case of thou  (used as an attributive adjective before a noun beginning with a consonant sound): thy table.  
"Thy" is an old English word meaning "your" and was used when talking to one person. "Thy" may still be used in parts of Yorkshire.