Thou, thee, thy, thine. Sounds old-timey and super formal, right? Well, you'd be right about old-timey, but not formal. In early modern England these were the singular informal forms of the second person personal pronouns, and people didn't just use them willy-nilly. In other words, they're archaic versions of "you, your, yours" that convey familiarity or indicate that the person being addressed is of similar or lesser social rank. If you were a farmer, you wouldn't walk up to a king and say "thy crown looks good today, Sire" because it would be disrespectful. Hint hint, fantasy writers. Eventually, use of the singular informal became synonymous with insult; only a few groups of people, like Quakers, continued to use it for religious or literary purposes.