The Roundheads were a group in the English Civil War who promoted a Republican commonwealth instead of a monarchy. The term was actually pejorative, and members of this group would not have used it to describe themselves. “Roundhead” is more catchy than “Parliamentarian,” their official title, and many historians of the English Civil War still refer to the parliamentary faction by this name.
The English Civil War was an important event in British history, marked by a major uprising against the traditional system of monarchy in Britain. The uprising was led by the Parliament, which was unhappy with many of the activities of King Charles I, especially when these activities involved abrupt dissolution of the Parliament when it appeared to be going against him. The Parliamentarians were involved in a series of extensive political moves and three major conflicts that came to be collectively known as the English Civil War.
Many of the Roundheads were Puritans who dressed and groomed modestly, in marked contrast to the styles in vogue at the court of King Charles I. This is probably where the slang term originated, contrasting their closely-shorn heads with those of their long, lustrous locked opponents. Although the hairstyle issue may seem petty, it is a valuable illustration of the religious and social differences between the two factions, and Puritan ideals were an important part of the Parliamentarian ethic.
The origins of the name are somewhat unclear, but it seems to date from around 1641, right before the outbreak of the English Civil War, and it was definitely meant to be offensive. The Royalists used the term as a put-down for their opponents, and among Parliamentarians, the word was not uttered. In fact, members of the Puritan New Model Army, which played an important role in many Civil War victories, could be severely punished for using this term.
Despite being Puritans, the Roundheads were not above a little name calling themselves. They coined the term “Cavalier” to describe their opposition on the side of the King, meaning to suggest that the Royalists were vain, reckless, and arrogant. Unlike their opponents, the Cavaliers ended up embracing their supposedly offensive title. Unfortunately for the Cavaliers, the Roundheads ended up being the superior military and political force in the English Civil War, ultimately masterminding an execution of King Charles I and establishing a new republic. The Republican commonwealth was shortlived, however, and in 1660, the monarchy was restored.