An Etymological Dictionary of the German Language/rennen
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rennen, vb., ‘to run,’ from MidHG. and OHG. rennen, prop. ‘to cause to flow, chase, drive,’ espec. ‘to make a horse leap, burst,’ hence the reflexive meaning of the ModHG. word; corresponding to OSax. rennian, Goth. rannjan, which are factitives of rinnen.