The Roundheads was the nickname given to the supporters of Parliament during the English Civil War. Their enemies, the Royalist supporters of King Charles I, were nicknamed the Cavaliers.
Some of the Puritans, but by no means all, wore their hair closely cropped round the head, and there was thus an obvious contrast between them and the men of courtly fashion with their long ringlets.
During the war and for a time afterwards Roundhead was a term of derision - in the New Model Army it was a punishable offence to call a fellow soldier a Roundhead. The name remained in use to describe those with republican tendencies until after the Glorious Revolution of 1688.
Roundhead appears to have been first used as a term of derision, towards the end of 1641 when the debates in Parliament on the Bishops Exclusion Bill were causing riots at Westminster. One authority says of the crowd which gathered there: "They had the hair of their heads very few of them longer than their ears, whereupon it came to pass that those who usually with their cries attended at Westminster were by a nickname called Roundheads."
John Rushworth (Historical Collections) is more precise. According to him the word was first used on 27 December 1641 by a disbanded officer named David Hide, who during a riot is reported to have drawn his sword and said he would "cut the throat of those round-headed dogs that bawled against bishops."
The principal advisor to Charles II, the Earl of Clarendon (History of the Rebellion, volume IV. page 121) remarks on the matter: "and from those contestations the two terms of 'Roundhead' and 'Cavalier' grew to be received in discourse, . . . they who were looked upon as servants to the king being then called 'Cavaliers,' and the other of the rabble contemned and despised under the name of 'Roundheads.'"
Richard Baxter ascribes the origin of the term to a remark made by Queen Henrietta Maria at the trial of the Earl of Strafford; referring to John Pym, she asked who the roundheaded man was.