Bosom is not an anatomical term, but refers to an area, or an enclosure formed by the chest and the arms of a person. An individual lying in the bosom of a friend can hear the beating of the friend’s heart and the breathing of his lungs. The friend’s arms, almost by reflex action, encircle such a person with love and protection.
The word “bosom” is found many times in the Bible and usually with the connotation of warm affection and safety. In the parable of Lazarus and the rich man (Luke 16:22, 23), Abraham’s bosom is represented as a place of complete felicity, where Lazarus feels the security and love of a child hugged by the arms of its father.
John 1:18 speaks of Jesus “in the bosom of the Father.” In that passage we are aware of a new element, and that is the thought of reward and honor. Jesus has completed His mission and is receiving gratitude and honor.
Occasionally the word has a sinister meaning: “Anger lodges in the bosom of fools” (Eccl 7:9). At times it relates to bribery: “A bribe in the bosom [averts] strong wrath” (Prov 21:14). It occurs also in a vengeful prayer—“Return sevenfold into the bosom of our neighbors the taunts with which they have taunted thee, O Lord” (Ps 79:12).
International Standard Bible Encyclopedia (1915)
Figurative: In a figurative sense it denotes intimacy and unrestrained intercourse (Ge 16:5; 2Sa 12:8); tender care and watchfulness (Isa 40:11); closest intimacy and most perfect knowledge (Joh 1:18); "into their bosom" (Ps 79:12) indicates the bosom as the seat of thought and reflection.