An old-fashioned word for "your" — the possessive form of "thou" (the old word for "you" when talking to one person). You will not hear this in normal speech today. You meet it in three places: the Bible ("Honour thy father and thy mother"), Shakespeare and other old poetry, and hymns or prayers. There is a small rule that goes with it, the same rule as "a" and "an": "thy" comes before a word starting with a consonant sound (thy book, thy hand), and "thine" comes before a vowel sound (thine eyes, thine own heart). "Thine" also works alone, the way "yours" does: "This book is thine." One thing that surprises people: "thou" and "thy" were once the WARM, close words, used to family, children, close friends, and God — while "you" was the polite, distant word for a stranger or a superior. So when the Bible says "thy," it is not being grand and formal; it is being intimate. English later dropped the whole warm form and kept only the polite one, which is why "thy" now sounds grand to us — exactly backwards from what it meant. Modern writers sometimes use it on purpose to sound ancient or holy, and jokey signs and fantasy games use it for flavour, often getting the grammar wrong.