Undo \Un*do"\, v. t. [AS. und?n. See 1st {Un-}, and Do to perform.] 1. To reverse, as what has been done; to annul; to bring to naught. [1913 Webster] What's done can not be undone. --Shak. [1913 Webster] To-morrow, ere the setting sun, She 'd all undo that she had done. --Swift. [1913 Webster] 2. To loose; to open; to take to piece; to unfasten; to untie; hence, to unravel; to solve; as, to undo a knot; to undo a puzzling question; to undo a riddle. --Tennyson. [1913 Webster] Pray you, undo this button. --Shak. [1913 Webster] She took the spindle, and undoing the thread gradually, measured it. --Sir W. Scott. [1913 Webster] 3. To bring to poverty; to impoverish; to ruin, as in reputation, morals, hopes, or the like; as, many are undone by unavoidable losses, but more undo themselves by vices and dissipation, or by indolence. [1913 Webster] That quaffing and drinking will undo you, --Shak. [1913 Webster]