From Guarenna or Varenna, in the county of Calais, in Normandy, whence they came into England with William the Conqueror. The primary sense of the word is to stop, hold, or repel, to guard, keep off.

Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.

WARREN. William de Warene, ov Warrena, who married Gundrada, a daughter of Willia7n the Conqueror, received great possessions in Sussex, Surrey, Norfolk, Suffolk, &c., and became progenitor of the Earls of Warenne and Surrey. His chief seat, anterior to the Conquest, was at Bellencombre, a small town in the arrondissemeut of Dieppe, in Normandy, on the little river Yarcnne. By this name the town itself was anciently known, until upon the ci-ection of a fortress upon an artificial mound, or heMus cumvlus, it received, from that circumstance, the appellation of Bellencombre. Arch. Journ., iii., 6. The Norman de Warenues were doubtless progenitors of many existing families of Warren ; but it must not be forgotten that the surname may ha■e a totally different source, namely, n-arren — which Bailey defines as, " a franchize or place privileged by the king for keeping conies, hares, partridges, pheasants, &c.;" though the phrase is now more commonly applied to a colony

Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.