The phrase "Where art thou" is an archaic way of saying "Where are you?"

• "Art" is an old English form of the verb "to be," specifically the second person singular present tense (you are).
• So, "Where art thou?" literally means "Where are you?"

This phrase is famously used in Shakespearean English, for example in Romeo and Juliet ("Wherefore art thou Romeo?"), which means "Why are you Romeo?" (or "Why do you have to be Romeo?"). If you have any more questions about archaic English or Shakespearean phrases, feel free to ask!