A propeller hat is a type of novelty hat that has one or two propeller blades on top that can spin horizontally. The hat is usually colorful in nature and visorless, though propeller hats with visors are becoming more common as well.
The propeller hat was developed by Ray Faraday Nelson in 1947. He got the idea when he and a group of friends were attending a regional science fiction convention. He and his friends wanted to take pictures mocking the covers of science fiction magazines and that's when he got the idea for the hat. The hat was later worn by George Young at a worldwide convention where it soon exploded in popularity.