"The hat itself was one of those flat-topped pork-pie jobs with a jay's feather stuck in the hat-band and Mr Wormwood was very proud of it."
A pork pie hat
As you can see from the picture, pork pie hats look like pork pies. They are made of felt and sometimes straw and look a bit like a fedora or a trilby except that they have a flattened top. They originate from the 19th century and were first worn by women before jazz musicians made them popular on the music scene.