Sorry, that was rude of me.
Although we associate the pronoun thou with high-fallutin’ fancy talk since nobody says it anymore, it was in practice used informally. Basically an anglicized equivalent of tu (it was originally spelled thu), as seen in French, Spanish and Latin, it would have been used to address a close friend, a child, or a subordinate.
If you want to fancifully address someone in an outdated way but keep it classy, you would use ye. This doubled as the second person plural in Middle English as well.