Thou is an old-fashioned, poetic, or religious word for "you" when you are talking to only one person. It is used as the subject of a verb. 你; 汝; 尔; 旧用词,现亦用于诗歌或宗教类语境
pronoun [人称代词]你;尔,汝(古时候的叫法)
'Thou art' is an archaic form of 'you are'. thou art是you are的古体。