Doek is Dutch for cloth, which was evolved into the English word "duck" in reference to sail canvas. Duck was typically made from cotton or linen (flax), with some use of hemp. These natural fibers have poor resistance to rot, UV light and water absorption.
Linen is stronger, but cotton is lighter. Linen was the traditional fiber of sails until it was supplanted by cotton during the 19th century. As sail size grew linen was too heavy to be practical so cotton became more popular