Either a corruption of brother, a modification of bud, or a borrowing from Pennsylvania German Bub or Southern German Bub (as was spoken in various communities in America before the early 20th century), ultimately thought to be derived from Proto-Germanic *bō- and thus cognate to English boy and babe.
West Frisian bobbe, German Bube, dialectal Swedish babbe, English babe, baby, and bully, Dutch boef, Middle Low German bôve, and Icelandic bófi.