An Etymological Dictionary of the German Language/Annotated/borgen
borgen, verb, ‘to borrow, lend,’ from Middle High German borgen, Old High German borgên, originally ‘to watch over, spare a person,’ then ‘to remit him his debt, to borrow’; also ‘to be surety for something’; similarly Anglo-Saxon borgian, ‘to protect’ and ‘to borrow,’ English to borrow. Since the meaning ‘to watch over’ underlies both borgen, ‘to borrow,’ and bürgen, ‘to be responsible,’ the word may be compared with Old Bulgarian brěgą, ‘I take care of.’ The root may have been Teutonic borg-, pre-Teutonic bhergh-; perhaps bergen is to be connected with the same root.