Wiktionary
n. A flat-topped, round, man's hat popular among musicians of jazz, blues, and ska, so called for its similarity in appearance to a traditional pork pie and made famous by the actor http://en.wikipedia.org/wiki/Buster%20Keaton and by the jazz musician http://en.wikipedia.org/wiki/Lester%20Young.
Wikipedia
For the hat sometimes known as a pork pie hat, see Smoking cap.
A pork pie hat is one of three or four different styles of hat that have been popular in one context or another since the mid-19th century, all of which bear superficial resemblance to a culinary pork pie dish.
Usage examples of "pork pie hat".
No one was about except a large woman in a pork pie hat giving her two Rotweillers a run.
When I had first entered the hillside barn I had been surprised to see a little bright-eyed old man in a pork pie hat settling down comfortably on a bale of straw.
His blue pork pie hat was set very square on his head and chalk-white hair showed under it.
Rather heavy set, wearing a brown suit and a dark pork pie hat with a gay band.