What is the X in X-mas?
Old English bosm "breast; womb; surface; ship's hold," from West Germanic *bosm- (cf. 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.
bosom bos·om (buz'əm, bōō'zəm)
The chest of a human.
A woman's breast or breasts.
In the East objects are carried in the bosom which Europeans carry in the pocket. To have in one's bosom indicates kindness, secrecy, or intimacy (Gen. 16:5; 2 Sam. 12:8). Christ is said to have been in "the bosom of the Father," i.e., he had the most perfect knowledge of the Father, had the closest intimacy with him (John 1:18). John (13:23) was "leaning on Jesus' bosom" at the last supper. Our Lord carries his lambs in his bosom, i.e., has a tender, watchful care over them (Isa. 40:11).