ROUNDHEAD, n. A member of the Parliamentarian party in the English civ… — The Devil's Dictionary
SAPIR 861
ROUNDHEAD, n. A member of the Parliamentarian party in the English civil war--so called from his habit of wearing his hair short, whereas his enemy, the Cavalier, wore his long. There were other points of difference between them, but the fashion in hair was the fundamental cause of quarrel. The Cavaliers were royalists because the king, an indolent fellow, found it more convenient to let his hair grow than to wash his neck. This the Roundheads, who were mostly barbers and soap-boilers, deemed an injury to trade, and the royal neck was therefore the object of their particular indignation. Descendants of the belligerents now wear their hair all alike, but the fires of animosity enkindled in that ancient strife smoulder to this day beneath the snows of British civility.
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