A pillbox hat is a small woman's hat with a flat crown and straight, upright sides, and no brim.
Historically, the pillbox was also military headgear, often including a chin strap, and can still be seen on ceremonial occasions in some countries, especially former members of the Commonwealth. 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 manufacture of the traditional headdress worn by members of virtually all Gurkha Regiments.[1] During the late Roman Empire, the pillbox, then known as the pilleus or "Pannonian cap" was worn by Roman soldiers.
In military terms the pillbox is a round defence structure for ground troops, popularized in World War I by the British.[2] Pillbox fortifications still stand today and are particular prevalent in Wales, such as those at Nant Ffrancon.