'roundhead' originates from English, specifically referring to the supporters of the Parliament during the English Civil War, who were derogatorily called 'roundheads' due to their short haircuts.

'roundhead' was used during the 17th century to describe the Parliamentarians, and it has remained largely unchanged in modern English.

Initially, it meant 'a person with a round head or short hair,' but over time it evolved into its current meaning of 'a supporter of the Parliamentarians during the English Civil War.'