An Etymological Dictionary of the German Language/borgen
borgen, vb., ‘to borrow, lend,’ from MidHG. borgen, OHG. borgên, orig. ‘to watch over, spare a person,’ then ‘to remit him his debt, to borrow’; also ‘to be surety for something’; similarly AS. borgian, ‘to protect’ and ‘to borrow,’ E. to borrow. Since the meaning ‘to watch over’ underlies both borgen, ‘to borrow,’ and bürgen, ‘to be responsible,’ the word may be compared with OBulg. brěgą, ‘I take care of.’ The root may have been Teut. borg-, pre-Teut. bhergh-; perhaps bergen is to be connected with the same root.