bub (n.)

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 is attested from 1920.

familiar address or nickname for a male, 1873 (in a South Carolina context), Southern U.S. slang, said to be a corruption of brother.

"a woman's breast," 1680s, of uncertain origin. Compare boobs.