Green’s Dictionary of Slang

the female breasts.

a.1790		C. Dibdin ‘Buxom Nan’ in Collection of Songs II 72: Tant masted all, to see who’s tallest, / Breastworks, top gant-sails, and a fan; / Mesmate, cried I, more sail than ballast, / Ah still give me my buxom Nan.
1826		C.M. Westmacott Eng. Spy II 368: The frigate yonder with the brown breast works, and she with the pink facings, look something like privateers.
1935		J. Conroy World to Win 156: Did you lamp them breastworks? Oi! Oi!
1946		H.A. Smith Rhubarb 93: Harold Harper, the newspaperman who forged a career by charging across the breastworks of a nation.
1950		J.T. Farrell ‘Milly and the Porker’ in Amer. Dream Girl (1950) 196: He told himself that if she bent over near his table he could see her breastworks.
1961		G.L. Coon Meanwhile, Back at the Front (1962) 180: Overshadowed by breastworks of such magnitude and design as to stagger credulity.