Definition of "breastwork"

Before long we had flung enough sand-bags into place to make a low breastwork behind which the few men who were on this side of the position could lie down and fire.

1938 April, George Orwell [pseudonym; Eric Arthur Blair], chapter VII, in Homage to Catalonia, London: Secker & Warburg

A cannonproof breastwork, built during the previous war, extended along the beach from the hills to the rocks.

1983, Richard J. Hargrove, General John Burgoyne, page 26

The Independencia is a two-turreted, breastwork ship of 9,000 tons displacement. […] The central breastwork is 130 feet in length at the top of the belt, and extends to the upper deck, 11 feet above the water-line. This breastwork incloses the boiler and engine hatches, the scuttles to magazines and shell-rooms, the principal openings for ventilation, and the two turrets.

1878, J. W. King, Report of Chief Engineer J. W. King, United States Navy On European Ships of War and Their Armament, Naval Administration and Economy, Marine Constructions and Appliances, Dockyards, etc., etc.‎, Washington, page 287

A swarthy boy opened a book and propped it nimbly under the breastwork of his satchel. He recited jerks of verse with odd glances at the text: […]A figurative use.

1922 February, James Joyce, “[Episode 2]”, in Ulysses, Paris: Shakespeare and Company, […]