"structure (in a building, bridge, etc.) in the shape of a curve that stands when supported only a the extremities," c. 1300, from Old French arche "arch of a bridge, arcade" (12c.), from Latin arcus "a bow" (see arc (n.)). Replaced native bow (n.1).
Originally architectural in English; transferred by early 15c. to anything having a curved form (eyebrows, feet, etc.). The commemorative or monumental arch is attested in English from late 14c. Compare Middle English Seinte Marie Chirche of the Arches (c. 1300) in London, later known as St. Mary-le-Bow, site of an ecclesiastical court, so called for the arches that supported its steeple (the modern church is by Sir Christopher Wren, rebuilt after the Great Fire of 1666).