noun

• (anatomy, somewhat dated) The breast or chest of a human (or sometimes of another animal).
  "Serene, smiling, enigmatic, she faced him with no fear whatever showing in her dark eyes.[…]She put back a truant curl from her forehead where it had sought egress to the world, and looked him full in the face now, drawing a deep breath which caused the round of her bosom to lift the lace at her throat."

verb

• (intransitive) To belly; to billow, swell or bulge.
  "Just above the recess the cliff bosomed out with a full swell for some two or three feet, effectually preventing any one’s looking down into the nest from above […]"
• (transitive) To belly; to cause to billow, swell or bulge.
  "1822, James Hogg, The Three Perils of Man, London: Longman, Hurst, Rees, Orme & Brown, Volume 3, Chapter 12, pp. 440-441, I looked again, and though I was sensible it must be a delusion brought on by the stroke of his powerful rod, yet I did see the appearance of a glorious fleet of ships coming bounding along the surface of the firmament of air, while every mainsail was bosomed out like the side of a Highland mountain."