Old English bosm "breast; womb; surface; ship's hold," from West Germanic *bosm- (cognates: Old Frisian bosm, Old Saxon bosom, Middle Dutch boesem, Dutch boezem, Old High German buosam, German Busen "bosom, breast"), perhaps from PIE root *bhou- "to grow, swell," or *bhaghus "arm" (in which case the primary notion would be "enclosure formed by the breast and the arms"). Narrowed meaning "a woman's breasts" is from 1959; but bosomy "big-breasted" is from 1928. Bosom-friend is attested 1580s; bosom buddy from 1920s.