A derisive term from The English Civil War (and in used around 1642), a Roundhead was a supporter of Parliament (and thus against the Monarchy of King Charles I). While not all Roundheads were “Commoners”, the bulk of The Parliamentarian Army was. Officers were mostly vested Nobility who supported both Thomas Fairfax and Robert Devereux (and later Oliver Cromwell). Many Historians have various “origin stories” regarding the actual development of the term. One most common, is that most Roundheads shaved or cropped their hair extremely short. This was seen as a slight to the common fashion thread of long hair with curled “ringlets” (seen in popularity with Royalty and it's Supporters).
The lack of hair gave a clear view to the entirety of one's “rounded” head (hence the term). Yet another variation, comes from the distinctive shape of the war helmets that The Parliamentarian Army wore (akin to the Lobster Pot Helmet) – with or without face visor. Under Cromwell (later in The War), the term was seen as offensive (usage of which in his “New Model Army”) could yield flogging. The term “Roundhead” also carried over to all Protestant religious sects (such as Puritans, Calvinists, etc.) it has also been put forward by some Historians, that the term would eventually “evolve” into the political derisive term “Whig” (or one who is a member of a Radical political party).