The Gullwhacker was a piece of knotted rope used by Mariel Gullwhacker as a weapon. Gabool the Wild bound Mariel to a piece of wood with this rope before throwing her in the ocean. When she awoke on the beach at the edge of Toadlands, Mariel used this rope to chase away gulls who attacked her, thus giving the weapon its peculiar name.
The Gullwhacker's characteristic as a fighting device stemmed from a set of complex knots giving its striking end greater weight and hardness, making it a flexible club of sorts. It was especially effective when wet, as this added more weight, and was used as a deterrent against attackers and as a means of disarming them. It was often twirled like a lasso, or swung like a flail.
During the mousemaid's temporary memory loss, the Gullwhacker acted as her most familiar and precious object. She even included its name it as part of her own name, calling herself Storm Gullwhacker. After her recovery, Mariel used the weapon extensively in her adventures. Later, she hung the original Gullwhacker as the tolling rope for the Joseph Bell of Redwall Abbey.
On further adventures, Mariel devised similar knotted ropes with the same purpose as the Gullwhacker. A gullwhacker could then be any length of rope tied with complex knots designed to make it into a weapon.
The Gullwhacker may have been based on a real-life nautical knot known as a "Monkey's Fist," a large, spherical knot that was designed to weight the end of a heaving line, but was sometimes used as a bludgeoning weapon by sailors.