The word "thy" is an archaic form of the word "your." It is used in English, particularly in historical or biblical contexts, to indicate possession and address someone directly. "Thy" is singular and informal, typically used when addressing someone of equal or lower status.
The word "thy" comes from Old English "þīn," which is a possessive pronoun meaning "your." It has its roots in the Proto-Germanic "*þīnaz," which is also related to similar words in other Germanic languages.