The Thief Knot resembles the Square Knot (Reef Knot) except that the ends of the Thief Knot come off opposite sides of the knot. In the Square Knot, they come off the same sides. The Thief Knot has no strength and will slip under tension very easily.
It is said that sailors would tie their sea bags closed with the Thief Knot. If another sailor went through the bag, it was assumed that the thief would retie the bag with a common Square Knot. This would alert the bags owner that his bag had been opened, hence the name of this knot.