A watch cap is a knitted hat worn by e.g. a sailor or soldier on duty (hence "On Watch") - designed to provide warmth in cold weather. The hat is simple in its construction, although there are many variants. Historically, the knitted woolen hat was a common form of headgear for sailors, fishermen, hunters and others who spent their working day outdoors from the 18th century onwards - and are still used for this purpose in Canada, Scandinavia and other cold regions of the world.
Army Star has a large and varied selection, fleece or knitted, and in muted or funky colors, so when looking for watch caps this is the place to be.