a member or adherent of the Parliamentarians or Puritan party during the civil wars of the 17th century (so called in derision by the Cavaliers because they wore their hair cut short).
(English history) a supporter of Parliament against Charles I during the Civil War Compare Cavalier
Word Origin
referring to their short-cut hair
Word Origin and History for Roundhead
n.
"adherent of the Parliamentary party in the English Civil War," 1641, so called for their custom of wearing the hair close-cropped, in contrast to the flowing curls of the cavaliers.