"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
Creative Commons Attribution Share AlikeA 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.