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. 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.