Thou is an old English word meaning “you” when speaking to one person. For hundreds of years, English speakers used thou the way we now use you, but only when talking to a single person. When addressing someone formally or respectfully, they would use a different word: ye or you.

By the 1600s, English speakers began using you for everyone, and thou gradually disappeared from everyday speech. Today, you mainly encounter thou in older texts like Shakespeare's plays, the King James Bible, or poetry from centuries past. When reading these works, whenever you see thou, thee, thy, or thine, they all refer to “you” in various grammatical forms.

Some religious communities still use thou in prayers, preserving the traditional language even though it sounds formal to modern ears. When a Shakespearean character says “I love thee,” they're using the familiar, affectionate form of “you,” even though to our ears it sounds stiff and old-fashioned.