1850, American English, possibly an alteration of brother, or from British colloquial butty "companion" (1802), itself perhaps a variant of booty in booty fellow "confederate who shares plunder" (1520s). But butty, meaning "work-mate," also was a localized dialect word in England and Wales, attested since 18c., and long associated with coal miners. Short form bud is attested from 1851. Reduplicated form buddy-buddy (adj.) attested by 1952, American English.

Lenny Kent, a long-time fave here, is really in his element. ... After four weeks here he's got everyone in town saying, "Hiya, Buddy, Buddy" with a drawl simulating his. [Review of Ned Schuyler's 5 O'Clock Club, Miami Beach, Fla., Billboard, Nov. 12, 1949]

Buddy system attested from 1920.

1925, usually with up, from buddy (n.); perhaps originally U.S. underworld slang. Related: Buddied; buddying.

Old English bosm "breast; womb; surface; ship's hold," from West Germanic *bōsmaz (source also of 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"), or possibly a word from a substrate language.

Bosoms in the narrowed or euphemistic meaning "a woman's breasts" is from 1959; bosomy "big-breasted" is from 1928 (earlier of rolling hills, etc.). Bosom-friend is attested 1580s; bosom buddy from 1924. Abraham's bosom "the abode of the blessed" is from Luke xvi.19-31.