A beanie is a small, round, brimless hat. It was fashionable for boys to wear beanies in the first half of the 20th century.
Traditionally, a beanie was a flat, close-fitting hat without a brim, sometimes also called a skullcap. In the early 1900s, adult workers and young men commonly wore beanies, which eventually evolved into today's baseball cap. The hat was probably named beanie after the bean-sized, cloth-covered button in the center of the cap. In the UK, a beanie is simply a knitted stocking cap.