The phrase "Thou art" is an old-fashioned expression that means "you are." It is a formal way of addressing someone that was commonly used in earlier times. Some common synonyms for this phrase include "you're," "you exist," and "you stay." Other synonyms include "you live," "you act," and "you dwell." These words can be used interchangeably with "thou art" depending on the context and tone of the speaker. While the phrase "thou art" may not be used as often today, understanding its synonyms can help broaden your vocabulary and enhance your ability to communicate effectively.