Thou is a second person singular pronoun of the English language. Thou is the nominative case; the oblique/objective (functioning as both accusative and dative) is thee, and the genitive is thy or thine.
In modern English thou continues to be used only in some of the regional dialects of England, some religious contexts (referring to God when capitalized) and in certain specific phrases, e.g. "holier than thou", "fare thee well". Otherwise, its contemporary use is an archaism. Find out more...