An Etymological Dictionary of the German Language/Busen
Busen, m., ‘bosom,’ from the equiv. MidHG. buosen, buosem, OHG. buosam, buosum, m..; comp. OSax. bôsm, Du. boezem, AS. bôsm, E. bosom; in East-Teut. (Goth., Scand.) the corresponding word (Goth. *bôsma-) is wanting. It may perhaps be allied to Bug, MidHG. buoc, ‘arm, shoulder’ (pre-Teut. bhâghu-); but since a pre-Teut. bhâghsmo, bhâksmo- does not occur in the cognate languages, nothing can be cited in favour of that explanation; at all events, Busen is not allied to biegen.