The word "thee" is an archaic or poetic form of the pronoun "you." It is used in English to address one person in a familiar or intimate way. This form is largely associated with older literature and religious texts, such as the King James Bible.
"Thee" comes from Old English "þe," which was used as the second-person singular pronoun. It can be traced back to Proto-Germanic *þū, which also led to similar forms in other Germanic languages.