The cutlass was a short, broad saber or slashing sword. It was the Close-Range Weapon of the Pirate.
The Cutlass was a short, slightly curved sword sharpened on the cutting edge. The hilt often had a solid cupped or basket shaped guard.
The cutlass was best known as the sword of choice for sailors, likely because it was robust enough to hack through ropes, canvas, and wood, but was also short enough to use it relatively close quarters, such as during a boarding operation, in the rigging, or below deck. Another advantage was the simplicity of use, since it required less training than a rapier or small sword.
Pirates used these weapons for intimidation as much as for combat, often needing no more than to grip the hilts of their cutlasses to induce a crew to surrender, or using the flat part of the blade for striking captives to force their compliance or responsiveness to interrogation.
By the 1800s the cutlass became a strait sword. As naval technology and gun technologies improved the cutlass eventually died off, especially with WWI showing the pointlessness of swords once machine guns were introduced.