Historically, the pillbox hat was military headgear, often including a chin strap, and it can still be seen on ceremonial occasions in some countries, especially those of the Commonwealth of Nations. For example, the Royal Military College of Canada dress uniform includes a pillbox hat. A pillbox cap, also referred to as a kilmarnock, is a modern version of the traditional headdress worn by members of virtually all Gurkha regiments. During the late Roman Empire, the pillbox, then known as the pilleus or "Pannonian cap" was worn by Roman soldiers.
The name of the headgear derives from a pillbox, a round defence structure for ground troops, popularized in World War I by the British. Pillbox fortifications still stand today and are particularly prevalent in Wales, such as those at Nant Ffrancon.