1911 Encyclopædia Britannica/Crenelle

CRENELLE (an O. Fr. word for “notch,” mod. créneau; the origin is obscure; cf. “cranny”), a term generally considered to mean an embrasure of a battlement, but really applying to the whole system of defence by battlements. In medieval times no one could “crenellate” a building without special licence from his supreme lord.