"fieldwork thrown up breast-high for defense," 1640s, from breast (n.) + work (n.) in "fortification" sense. Old English had breostweall in same sense.
Entries linking to breastwork
breast n.
Old English breost "mammary gland of a woman, bosom; the thorax or chest, part of the body between the neck and the belly; mind, thought, disposition," from Proto-Germanic *brust-/*breust- "breast" (source also of Old Saxon briost, Old Frisian briast, Old Norse brjost, Dutch borst, German brust, Gothic brusts), perhaps literally "swelling" and from PIE root *bhreus- "to swell, sprout" (source also of Middle Irish bruasach "having a broad, strong chest," Old Irish bruinne "breast").
The spelling conforms to the Scottish and northern England dialectal pronunciation. The figurative sense of "seat of the emotions and affections, repository of designs and secrets" was in Old English. Breast-plate "armor for the front of the body" is from late 14c. Breast-pump is from 1821.
work n.
Old English weorc, worc "something done, discrete act performed by someone, action (whether voluntary or required), proceeding, business; that which is made or manufactured, products of labor," also "physical labor, toil; skilled trade, craft, or occupation; opportunity of expending labor in some useful or remunerative way;" also "military fortification." This is from Proto-Germanic *werka- "work" (source also of Old Saxon, Old Frisian, Dutch werk, Old Norse verk, Middle Dutch warc, Old High German werah, German Werk, Gothic gawaurki), from PIE *werg-o-, a suffixed form of the root *werg- "to do."
The meaning "physical effort, exertion" is from c. 1200; that of "scholarly labor" or its productions is from c. 1200; the meaning "artistic labor" or its productions is from c. 1200. The sense of "labor as a measurable commodity" is from c. 1300. The meaning "embroidery, stitchery, needlepoint" is from late 14c.
Work of art attested by 1774 as "artistic creation," earlier (1728) "artifice, production of humans (as opposed to nature)." Work ethic recorded from 1959. To be out of work "unemployed" is from 1590s. To make clean work of is from c. 1300; to make short work of is from 1640s.
Proverbial expression many hands make light work is from c. 1300. To have (one's) work cut out for one is from 1610s; to have it prepared and prescribed, hence, to have all one can handle. Work in progress is from 1930 in a general sense, earlier as a specific term in accountancy and parliamentary procedure.
Work is less boring than amusing oneself. [Baudelaire, "Mon Coeur mis a nu," 1862]