The cloth covered button on the crown is about the size of a bean seed and may be the origin of the term “beanie” Some academics believe that the term is instead derived from a type of headgear worn in some medieval universities. The yellow hats (“bejaunus,” that is, “yellowbill” later “beanus,” a term used for both the hats and the new students) evolved into the college beanies of later years.
According to the Oxford Dictionary, the etymology is uncertain, but probably derives from the slang term “bean”, meaning “head”. In New Zealand and Australia, the term “beanie” is normally applied to a knit cap known as a tuque in Canada and parts of the US, but also may apply to the kind of skull cap historically worn by surf lifesavers and still worn during surf sports. The non-knitted variety is normally called a “cap” in other countries.
Thank you Wikipedia!