Thy is an old-fashioned English word that means “your.” You'll find it in older books, prayers, historical documents, and especially in the King James Bible, which was translated over 400 years ago. When someone in an old story says “thy sword” or “thy courage,” they're simply saying “your sword” or “your courage.”

The word was part of everyday English until about the 1600s, when people gradually stopped using it in normal conversation. Today, we mostly encounter thy when reading classic literature like Shakespeare's plays, reciting traditional prayers like the Lord's Prayer (“Thy kingdom come, thy will be done”), or singing old hymns and patriotic songs.

Writers sometimes use thy today when they want their words to sound formal, solemn, or old-timey. A fantasy novel might have a wizard speak with thy to make him sound ancient and mysterious. When you see thy in a text, remember it's just an older way of saying something perfectly ordinary: “your.”

Related words from the same time period include thee (you, as an object), thou (you, as a subject), and thine (your, especially before a vowel sound, like “thine eyes”).