A tricorn is an old-fashioned men's hat with three points, or corners. If you've ever seen the famous portrait of George Washington crossing the Delaware River, you know just what a tricorn looks like.
The first few United States presidents wore tricorns, which became less popular toward the end of the eighteenth century. Before that, European men favored them because their turned-up brims kept rain off their faces and shoulders. The Greek word for "three" is treis, the root of the tri part of tricorn. Corn is in this example related to corner, and both words come from the Latin cornu, or "horn."